Domestic Solar water heating systems

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Domestic Solar water heating systems –which system will suit your home?
Solar water heaters (SWH) have been available for decades in South Africa, and yet have not
achieved the market penetration one would expect, even with the significant rebate Eskom
offers. I have heard comments like “they are too expensive” and “they are not reliable” etc, but if
you ask me, the real reason is simply the combination of a) an overwhelming number of suppliers
& system types and b) a surplus of technical waffle with its associated graphs, tables and pie
charts, and above all, c) misleading marketing propaganda.
This article will hopefully answer a few of the major questions concerning SWH’s, and empower
you to make a well informed decision as to what the most appropriate system is for you.
Everything in this article is based on research into peer reviewed journals, comprehensive local
market surveys carried out by the Project 90x2030 renewable energy team, interviews with
suppliers and manufacturers as well as my opinion as an Engineer (with a little common sense
thrown in for good measure).
What is a SWH system?
A SWH system is made up of a collector (the panel that collects solar energy), the storage tank
(geyser), and the control system. Before you go out and get quotes for a SWH system there are a
few key things you should understand in order to select a system that will really suit you.
Flat Plate or Evacuated Tube collector?
There are two main types of solar collectors available on the local market, namely the Evacuated
Tube Collector (ETC) and the Flat Plate Collector (FPC), Figure 1 and Figure 2. ETC collectors are
made up of a series of individual glass tubes and FPC collectors look like boxes covered with a
glass sheet.
Figure 1: ETC
Figure 2: FPC
I provide more detail about the advantages and disadvantages of either system at the end of this
article, but the bottom line with regards to collector type is that if you live in a warm climate, go
for a FPC. If you live in a cold or cloudy climate (most of SA is considered to be a warm climate),
and you don’t want your electrical backup to be used too often, go for a ETC.
Direct or Indirect heating system?
This basically refers to how the water you use is heated. Simply put, in an indirect system, the
water you use in your house is heated indirectly by means of a heat transfer fluid (HTF) in a heat
exchanger. In other words the HTF is heated up by the solar energy, and then further down the
system the heat is transferred to your water, typically inside the storage tank.
In a direct system, the actual water that is used in your house is circulated through the collector
and is therefore heated up directly by the solar energy. Direct water heating systems are not
recommended for cold areas, due to the potential for the water freezing in the collector and
causing damage.
In my opinion, if you prefer a system that is more or less hassle-free, you should probably opt for
an indirect system. These systems do, however, require that the HTF is replaced every few years.
Active or Passive (thermosiphon) control system?
Active systems use an electric pump and a computer to control the circulation of the water. The
two advantages of this are 1) the controller only allows water to be circulated through the system
when the temperature in the collector is higher than the storage tank, thus ensuring the water only
gains heat from the collector, and 2) a pump and controller reduces the risk of overheating in
times when the water users are away, and also reduces the risk (in direct systems) of freezing, both
of which would cause damage to the system.
Passive systems sometimes referred to as thermosiphon systems, use the properties of natural
convection to create circulation. This circulation, therefore, will only take place when the sun is
shining. But for this type of system, there are very specific criteria regarding the structure of your
roof and positioning of the system that need to be met. Many houses are perhaps not suitable for
thermosiphon systems, especially if you want the tank inside your roof. Having the tank inside the
roof not only provides additional insulation (reduced losses during night time) but may also be
more aesthetically pleasing. Passive systems should be cheaper and result in less maintenance
because they are simpler and require few mechanical parts. So if your roof allows, thermosiphon
systems are a good choice.
What size tank?
The rule of thumb is 50 litres a day per person. So for two people 150 litres is perfect and for three
people 200; for four people 250 or 300 litres would be ideal. If you can afford it, choose a slightly
bigger size for the added protection against a few rainy days.
Costs
The expected cost of a system is based on a variety of things and trying to give a ball park price
estimate is not easy since so many factors need to be considered.
I suggest you use what I have said here to choose a system type that you think is the most suitable
for your house and then approach a few suppliers with the same specs and compare their
quotes. At least this way you are comparing apples to apples.
Finally, most suppliers will be able to supply data on the expected energy output of their system,
use this data to calculate energy output per Rand, which will give you a good idea of what you
getting for your money. The higher the energy per Rand, the more cost effective the system is.
Eskom Rebate explained
The amount of money received as a rebate depends on the system’s efficiency. To qualify for the
Eskom rebate, the system needs to have passed the SABS and SANS tests for performance, quality
and safety. The more efficient the system, the higher the rebate. Some suppliers can also submit
their products for the SABS Mark Approval, which is a voluntary scheme and gives a vastly better
assurance of the quality of the product. Look out for suppliers which have passed this test, since
there are currently very few. The fact that a particular system qualifies for a rebate is a good
indication of its quality and effectiveness. I would personally avoid systems that have not
qualified,not because I think they are automatically of poor quality , but simply because if they
do qualify, it means they have been subjected to quality tests and passed.
Some suppliers will have better rebates than others, so what I suggest is when you get quotes,
compare prices before the rebate has been subtracted. This will give you an indication of whether
the agent is simply hiking prices to match a high rebate. You can expect the prices for suppliers
who do qualify to be higher than those that don’t, since the approval process is quite costly, and
the costs have to be incorporated. However there should be no difference between the capital
costs of two different suppliers that do qualify, regardless of the rebate amount. The highest rebate
amount currently in South Africa is about R8,200 (for a 150 litre system), so anything in that ballpark
is great.
Conclusion
Over the next few months I will be going into more detail regarding all the issues I have mentioned
in this article, and comprehensive cost analysis will be one of them. In the meantime I would
suggest you choose a particular set of specifications and then just compare quotes. And
remember to get the agent to inspect your site before quoting on installation, because installation
costs depend on site-specific conditions.
In a nutshell, if I had to recommend a particular system, it would be an indirect, active system,
and based on climate, either a ETC or a FPC. If you are trying to minimise costs, a FPC is fine
anywhere, but be sure to check for temperature protection on any system you go for.
I would only go for a system that qualified for an Eskom rebate because I could assume that the
system has met sufficient quality requirements and ultimately result in a lower capital cost. I will go
into more depth on each of the issues covered briefly here over the next few months, so watch
out for the upcoming articles if you want to know more. In the meantime read on if you want to
know a bit more about solar water heating.
Cheers, Gary
More detail on Solar Water heating systems
More on Flat Plate or Evacuated Tube collector:
There has been some contention regarding which is the most appropriate in South Africa, and
agents selling either type will have an extensive list of advantages of their own system, as well as a
list of disadvantages of the other. Regarding ETC, the most common one you will hear is that it is
more “efficient”. Even though this might be true, owing to the tube reducing convection and
conduction heat losses, the actual benefit of this is questionable. Efficiency of a solar collector is
complicated and is dependent of many things, a lot of which are site specific. Since the purpose
of this article is to un-complicate things, I will not be covering this here. Anyone interested can
contact me and I will point you to a few good books on the subject.
So, due to their higher efficiency, ETC’s perform better in colder conditions than FPC (FPC’s have
been found to be more efficient than ETC’s in warmer conditions), however this advantage is less
important in warmer climates. Most of South Africa is considered a warmer climate, and unless
you live in a region that is particular cold with long periods of cloud cover, ETC’s , in my opinion,
are not necessary. The main advantage of ETC technology is that they can achieve very high
temperatures, which is why they are often used in industry to create steam, but this is not
necessary for a domestic application. The main disadvantage would be that they are only highly
efficient as long as the tubes are actually operating. As soon as the vacuum is lost, i.e. broken, the
output is zero. To find out if all the tubes are still functioning, you have to actually inspect the
tubes, often this involves climbing onto the roof to get a good look. Some manufacturers include
easily visible indicators in the tubes for you to see from a distance whether they are still
operational.
FPC’s are the most common globally and are typically more suitable for low temperature
applications, i.e. domestic hot water, and are generally cheaper. Reviewing of Eskom supplier list
however, could not verify this, so it seems in South Africa there is little price difference. Most flat
plate collectors are very robust and have an expected lifespan of over 25 years. No need for
inspecting or changing broken tubes.
I need to add a note here - if you decide on a FPC, make sure that the collector surface is
painted with a “Selective” coating, not simply black paint.
More on direct or indirect systems:
In a direct system, the actual water that is used in your house is circulated through the collector
and is therefore heated up directly by the solar energy. In an indirect system, the water you use in
your house is heated indirectly by means of a heat transfer fluid (HTF) in a heat exchanger.
Direct water heating systems are not recommended for cold areas, due to the potential for the
water freezing in the collector and causing damage. This would not happen in an indirect system
since the HTF contains anti-freeze, similar to that in your car’s radiator. There are however, some
systems that take this into account and have measures in place to prevent it, but they often
require intervention by the user i.e. predicting when a cold spell is coming and draining the
system, or reversing the circulation. Direct systems are also not recommended for areas where the
water is extremely hard or acidic because scale deposits may clog or corrode the collectors1.
Scaling in direct systems should not be underestimated. To reduce scaling, the collector’s pipes
have bigger dimensions, which usually reduce performance.
More on Active or Passive systems:
It should be noted that passive systems can also have overheating protection, so you need to
check this. Active systems are more complicated, more expensive, harder to install and require (a
little) power to run. I would personally go for a system with a pump and controller. It costs a little
more, but in the long run it prevents potential damage to the system, and for something that will
probably last 20 years, I think this is important. However, this is not to say a thermosiphon system
not a good choice: if your roof allows, you prefer to not have any complicated computers and
use additional power, and you have ensured that the system has built in temperature protection,
then this is also a good option.
1
Kalogirou, S. 2004. Solar thermal collectors and applications. Process in Energy and Combustion Science, 30: 231 – 295.
February 2004
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