Solar Water Heating

Solar water heating is a relatively simple concept – the heat of the sun is captured using a collector panel and stored in a water tank.  There are of course many different systems configurations that one can choose, which complicates the issue somewhat, but as long as you understand the basics operation, advantages and disadvantages of each type of system, making an informed decision is relatively straightforward.  There are also many solar installers who will try and have you believe that solar water heating is very complicated, and during my investigations into the right system for our home, I met many people who I felt bordered on being con-artists.  Despite all my homework and choosing an installer who I felt confident with, our installation was not entirely problem-free, and thus the reason for this post is severalfold: to highlight the potentials of solar water, help you learn about the various options available, help steer you away from people who would have you believe that solar water heating is rocket science and that you require a “Rolls Royce” solution (this was the term actually used by one installer describing his ridiculously expensive offering, which in effect was no more than any of the standard offerings I had looked at) and also to learn from the mistakes of our installation.  Bear in mind what you want to achieve is reliable delivery of hot water to your taps with as little as possible dependence as possible on additional electrical heating of the water.

The basic principles of solar water heating

There are two main types of collectors – flat plate and evacuated tube collectors.  Flat plate collectors consist of  frame covered by toughened glass.  Water or the heat exchange fluid in the case of an indirect system flows through copper pipes in the collector which are heated by the sun.  This is the cheapest type of collector, but also unfortunately slightly less efficient and additionally is prone to freezing.  Dump valves are supposed to protect against freezing as can continuous slow cycling of water through the system when the ambient temperature drops to freezing levels (which unfortunately then wastes the heat in the system).  Alternatively the use of an indirect system utilising water plus antifreeze will protect against freezing.  Overheating using flat plate collectors is generally unlikely, because above a certain temperature, heat losses from the system equal the heating effect of the sun.

In an evacuated tube collector, each individual tube consists of an outer double-walled toughened glass tube containing a vacuum separated from an inner copper tube.  The inner copper tube is partially filled with water under a vacuum or alternatively a low boiling chemical and utilises evaporation and condensation to transfer captured heat from the sun to the water flowing through the manifold along the top header part  of the collector.   Evacuated tube collectors are somewhat more efficient than flat plate collectors, particularly when the rays of the sun are more oblique (i.e. early in the morning or late in the afternoon and also at higher latitudes), or when there is cloud cover.  This is because the vacuum layer in the outer double-walled glass tube minimises heat losses due to convection and conduction.  Under more direct sunshine, however, flat plate collectors are more efficient due to the larger area available for heat capture.  Evacuated tube collectors are more expensive than flat plate collectors, but are resistant to freezing.  They are, however prone to overheating and need to be protected from this through means of valves, partial shading or other means.

Water can be heated directly or indirectly.  In direct heating as the name implies, the water from the geyser is passed through the collector where it is heated and then returned to the hot water storage tank.  In an indirect system, the water never circulates in the system and instead another liquid, typically water plus non-toxic propylene glycol as an antifreeze, flows in a closed loop between the collector and the geyser where its heat is transferred to the water in the geyser.  Indirect heating is typically used in conjunction with flat plate collectors where the glycol in the system protects against freezing.  Such systems are more expensive to install and also because the glycol in the system degrades over time, it needs to be replaced periodically.
 
Water flow between the collector panel and hot water tank in solar heating systems can be passive or active.  A thermosyphon is a passive system and relies on the effect of density changes and the rising of hot water to circulate the water through the system  In such systems, the hot water tank needs to be placed above the collector panel and is thus often placed on the outside of the roof.  This close-coupled system is often seen where insufficient roof space is available for the water tank to be positioned inside the roof, but should there be sufficient space, a split system is possible with the collector on the outside of the roof and the hot water tank inside.  In contrast to this passive water circulation, water can be actively pumped through the system in a split system.  This is typically used when there is insufficient roof space available to place the water tank above the collector panel and where the hom-owner does not want an unsightly water tank on his/her roof.  Such systems are typically powered by means of a small solar panel, but can be powered from mains electricity as well.  Although to an extent this counters the electricity savings from solar water heating, the pumps are small and consume little electricity.  Pumped systems are more efficient at water heating than passive systems, but the presence of a pump also means that the system is more complicated, more expensive and has more components that can fail.  Pumps can also be used in conjunction with flat plate collectors to help protect against freezing.

Numerous combinations of collector panel, heating type and water flow are possible and include direct pumped retrofits of existing geysers, split active direct and indirect systems, split thermosyphon systems and close-coupled (on-roof) thermosyphon systems depending on the budget and requirements for the installation.

Our Installation

After doing a lot of reading and speaking to and receiving quotations from several different suppliers, we chose Gauteng Water Heating (GWH) to do our solar installation because they seemed honest, offered a good, reasonably priced system and did not try and pull the wool over my eyes in any way.  The choice was justified in the end because dealing with Jason and Tarryn was a pleasure and despite the problems we experienced, these were rectified with a smile.

Because of a very old low-pressure geyser, we decided to go for a completely new installation.  Our old geyser had a 200 litre capacity and based on this, as well as the guideline of allowing 50 litres of water per member of the household we decided to install a 200 l split-direct thermosyphon system with a  Power-zon flat-plate collector, a Solartherm geyser and a Geyserwise controller.  This was installed in 2011 and for the first 9 months of operation we experienced no problems.  However, early in winter the following year, on a cold but frost-free morning, the collector burst due to freezing (bear in mind that surfaces like the collector irradiate heat more rapidly than their surroundings and thus can drop below freezing even when the ambient temperature is above freezing).  Although the system was supposed to be protected against freezing by means of a dump valve, this did not function thus causing the failure.  Following discussions with the supplier of the collector, GWH decided to replace our collector with a Pacific Solar evacuated tube collector.  Although more efficient than a flat plate collector, we did not really notice a marked increase in water temperature.  However, during October of 2012 after going away for a long weekend, we returned home to find one of our bedrooms flooded and the ceiling damaged.  This was due to overheating of the geyser which caused the temperature/Pressure (TP) valve of the geyser to open with unfortunately no drainage pipe in place.  Despite a two week holiday the previous December, the system had not overheated due to its less efficient flat plate collector that had been installed at the time.  The third problem we have experienced with our installation has been a creeping one.  Our children have grown a little bigger over the past few years and Sula has reverted to hot water wash cycles in the washing machine (while being more environmentally friendly, cold cycles don’t always clean effectively).  Our demand for hot water has increased, and while this is okay for most of the year, during the cooler months the hot water supply is insufficient and we have to rely too much on electrical heating of the water.  Our dishwasher has a built-in heating element, but for washing up in the sink or washing floors, we always boil a kettle or two to supplement the water from the geyser.  Our geyser is also a long way from the kitchen, which means that five or six litres of water is also lost from the system before hot water is delivered to the taps.  We thus should have allowed installed a 250 or preferably 300 l hot water tank.  

Because of the big cost saving that can be achieved in doing so, the first prize in any installation is to convert your existing electrically heated geyser to a solar geyser by retrofitting the required collector and associated parts such as a solar pump top circulate the water through the system.  Consider the aesthetics of your installation.  A solar collector on a roof is unobtrusive, but a big water tank on top of the solar collector can be an unfortunate eyesore.  Simplicity is always best, and if there is sufficient height available in your roof space, a thermosyphon system means that there is no need for a pump to be installed, which always has the potential to fail.  The “rule of thumb” is to allow 50 litres of water per member of the household, but we have found that during the cooler months of the year this is insufficient and your should allow at least an extra 50 to 100 litre capacity above this.  Additional water also means that more hot water will be available if you have guests staying over or for other purposes.  Remember that the hot water tank effectively functions as a “battery” – the bigger the “battery” in your system, the more hot water you will be able to supply for longer.  Finally and most importantly, insulate all hot water pipes in your roof to reduce heat losses as much as possible.  Before making your decision, do your homework, understand the principles and what you require, speak to different suppliers and choose one who makes you feel confident.

Despite everything I have written above, my ultimate solar water heating system is not one that is readily commercially available.  I would like better control over the water flowing through the collector panel to prevent overheating.  Furthermore, if the water in the tank is insufficiently heated, I would like to heat only the water exiting the geyser on demand as it is wasteful to electrically heat a big tank of water.  Finally, although it is very useful, the Geyerwise controller is also very inflexible.

We are grateful that we chose to convert to solar water heating when we did and the system paid for itself long ago.  Now we enjoy a much reduced electricity bill and it adds value to our property.

* This article was written by my husband Clifford (the scientific half of this couple).