- Category: Technology
- Published on 01 January 2017
- Hits: 368
By James Green
It is illegal to install a solar water heating system that has not been tested and passed the SANS 1307 or doesn’t carry the SABS mark. Likewise, the SANS 10252, 10254 and 10106 need to be complied with for installation.
Within the SANS 1307 there are minimum and maximum performance criteria that need to be met to pass. The minimum is 5MJ and the maximum is 10MJ at 20MJ/m2 per day and per 50 litres of water in the storage tank or stated capacity. This in turn respectively means 2 778kWh minimum input per 100 litres and 5 556kWh maximum input per 100 litres.
Turning this into hot water means a minimum increase of 23.89°C in water temperature and a 47.78°C maximum increase per 100 litres.
Adding these temperature increases to cold water, using 16°C as an average in Johannesburg, results in 40°C minimum and 64°C maximum. In winter, when water temperatures to be heated are as cold as 11°C in Johannesburg, systems that are just meeting the SANS minimum criteria will not get hot enough to be used for cleansing unless electrical backup is used.
Installing an imported solar collector and bolting it onto an existing electrical geyser tank is a high-risk strategy. Not only may the performance be woefully inadequate — disappointing the consumer — but there also may be significant safety considerations.
The potential problems from overheating emerge in periods of stagnation, or when the hot water from the tank is not being used. This is likely to occur during heat waves or when the system has not been used for a number of days. Rather than being cold at 16°C, the starting point of the water being heated may be as hot as 60°C. Adding in the solar water heating performance on top of this could result in tanks and solar collectors becoming extremely hot and stressed, and components potentially failing.
Purely from a safety perspective, by law a temperature pressure (TP) relief valve must be fitted, which will open automatically if the tank temperature reaches 92–97°C. Water will be released through the valve and cold water will flow in, reducing the tank temperature. If the TP valve is overridden and the tank continues to heat, it will ultimately explode at around 135°C with tremendous force.
From a design perspective, the potential for overheating of water in the tank in a period of stagnation is most likely to occur in these instances:
- Integrated systems, where the solar collector is inserted into the tank (normally EVTs); and
- Thermosiphon systems, where the solar collector is of either a flat plate or an evacuated tube type that cannot be switched off (unless fitted with an additional special release valve).
The easiest systems to control are split forced circulation indirect systems, in that the pump (solar or mains) can be switched off when the tank temperature has reached a pre-set temperature (normally about 70°C).
While these split forced circulation indirect systems are arguably safer (if fitted with a solar controller) in that excessively hot water does not flow through the pipes to the taps, problems can still occur with EVT systems that use heat pipe risers into manifolds, or with flat plate collectors. In these instances, the solar collectors can reach temperatures of between 175°C and 250°C in prolonged stagnation. Rather than being a safety issue, they are likely to suffer a component failure over time.
These examples are valid reasons why ‘safety first’ should always be observed. Installing a system that has not met the requirements of the SANS 1307, including dry stagnation tests, is a potential recipe for litigation and even criminal charges.