How does it work?
In short, heat is collected by means of liquid being pumped through panels (collectors) that are exposed to sunlight or UV. Heated water is stored in an insulated tank for use. Most tanks have a ‘booster’ or electric element to get the water to the desired temperature should there not be adequate solar heat gain. Most of these boosters are electric elements placed inside the tank and can be switched off. Gas and heat pumps can also be used as ‘boosters’.
Here are the main issues or questions I get:
1. Water is heated at the opposite time to consumption, does solar make sense?
2. Luke warm water…
3. What about water pressure when using a solar system?
4. The geyser popping or exploding when water is not tapped when people are away,
5. Panels freezing where temperatures go below zero…
6. How do panels handle hail???
This is where a knowledgeable, experienced service provider is key as these issues are very simple to overcome if the system has been designed properly. I will get to the answers below, although they may be in no particular order.
Capacity is key when opting for a solar water heating system, to ensure an adequate amount of water is in storage for your requirements. In the typical family, kids bath in the evening and adults showering in the morning. Therefore it’s important that your storage tanks cater for this. Just like an electric geyser, solar is limited to the amount of hot water present in the tank. If the system has adequate capacity water will stay warm throughout the night to be used in the early morning. However rather than installing one large tank that can be diluted each time a hot water tap opens smaller tanks connected in series are a much better solution to provide adequate hot water. This also means that one can start with a smaller system and add to it as finances are available. Eskom recommends 50 liters per person per day, the national building regulations (SANS 10400 XA) recommends 85 – 115 liters per person per day. When installing an electrical geyser I would have said a 200 liter geyser for a family of four is adequate, bearing in mind that water is heating constantly. However for a solar application I would recommend 400 liters or 300 liters as a minimum for a family of four.
The basics about solar water heating systems
Direct VS Indirect
There are two methods of solar water heating- direct and indirect.
As the name suggests direct heating circulates the water used through the solar panel to heat it. Thus water used is heated directly by the sun.
The indirect heating method circulates a mixture of water and glycol, a liquid that has a much better heat transfer rate than water. The liquid is circulated through the collector and then through a perimeter coil in the water tank which then heats the water. My understanding is that the advantage of this system is that it can generate heat in lower sun / light conditions. However a disadvantage may be that it requires more maintenance as the glycol in some systems needs to be replaced annually. Some supplier’s claim that the glycol in their systems do not have to be replaced, It’s important to this if you are considering an indirect system.
To me a direct system makes more sense, who is going to remember that the glycol needs to be replaced?
High pressure VS low pressure
This can be somewhat complex, however very basically it’s important to know that there are high and low pressure solar systems. If you like showers that is like showering below a bucket and are not installing air rated water saving taps like Hans Grohe a low pressure system is adequate. However if you are using air rated taps or want a nice hard shower ensure that you opt for a high pressure system.
There are two types of collectors, flat plate and evacuated tubes.
Flat plate collectors are less expensive and require sunlight to generate heat; however these panels are more likely to freezing in winter if the system is not designed adequately.
Evacuated tubes are more expensive but only require UV to generate heat; obviously sunshine is beneficial and will add more UV.
If you have the budget opt for evacuated tubes.
The tank needs to be placed as close to the collector as possible. Most solar geysers are mounted horizontally on the roof with the collectors, which is very ugly and seems to make very little sense to me as this creates a greater water surface area, which leads to greater heat loss. In applications we have been involved in water tanks are placed vertically within the building to reduce this surface area and potential heat loss. Like regular geyser water tanks can and do burst so consider their placement.
Many installers promote solar pumps to circulate the liquid within the system and this seems like a great sustainable solution. However, as long as the sun shines the pump works so when the water exceeds the maximum temperature the tank can withstand the pump just keeps circulating and the tank eventually fails. Similarly when there is no sun the pump stops, so when the temperate drops the panels may freeze.
It’s important that the pump operates at all times and should have a battery back-up for load shedding (power failures if you are outside South Africa). In Addition the pump must be connected to two thermostats monitoring the tank and collector temperatures, to regulate its function. Once the water in the tank is the correct temperature the pump stops circulating water and so avoids overheating the tank. Similarly if the external temperature is too low the pump can pump warm water through the collectors to stop them freezing.
To answer the question about panels withstanding hail:
If you install a system that is SABS approved it will have passed a test to withstand 38mm diameter hailstones travelling at 140km’s an hour at 90 degrees to the tube. Flat plate collectors are more flexible than evacuated tubes so may fair better, however with evacuated tubes one only needs to replace the tubes that are broken not the entire panel. While the panels are susceptible to tennis ball size hail so is your roof, windows and walls.
When installing a solar water system with adequate capacity solar has a high capital outlay at the onset, possibly the highest of all the options. However in a country with some of the best sunshine in the world the running costs are so low that the average payback period is 2 – 4 years, thereafter your hot water is almost free, and you are no longer reliant on Eskom for a shower.
While solar systems generally tend to require the highest initial capital outlay they make absolute sense in the South African climate where we have miles of sunshine. However for the system to work effectively it needs ample capacity, be SABS approved and be installed by a reputable installer with adequate experience. In Africa, where we are not as regulated as the first world the ‘eco’ bandwagon is very appealing and many companies import Chinese product and do not have adequate experience.
I can recommend ‘Nupower’, speak to Adriaan Russow.
This post is a part of the ‘in hot water ‘series and should not be read in isolation.
Please see my conclusion on this issue of water heating.