African energy
29
New buildings’ energy efficiency
What does SANS 10400-XA 2 really mean for solar?
By
James Green
I monitor the electricity consumption on several electric geysers on a daily basis. One couple I know uses 6 – 8kWh per day in summer. In winter, their consumption jumps to 12 – 14kWh per day.
Naturally, there are a number of reasons for the higher kilowatt-hour consumption in winter: the water going into the geyser is colder( as low as 11 ° C) than in summer( as high as 20 ° C); they may shower at a slightly higher temperature( 41 ° C rather than 39 ° C), a little longer maybe; and then of course there is heat loss. The consumption is considerable on cold winter nights at 3 – 4kWh overnight, against 1 – 2kWh in summer.
Another couple I know uses far less. They are in the Cape and not in Johannesburg, and their consumption is 3 – 4kWh in summer and 7 – 8kWh in winter. They also tend to turn the hot water off, soap themselves, and then wash the soap off.
The point is that washing behaviour differs, and kilowatthour consumption will depend on income levels and regional differences.
SANS 10252 suggests about 75 litres from the geyser per person per day. Assuming a typical thermostat setting of 60 ° C and cold water at an average of 16 ° C, this would work out to about 135 litres at 40 ° C( a typical washing temperature). Using a high-pressure shower at 15 litres per minute, this works out at approximately nine minutes per person per 24 hours.
So how should SANS 10400-XA 2 be interpreted where in excess of 50 % of the electricity consumed by an electric resistance element needs to be replaced?
Using the two examples above, 50 % for the couple in Johannesburg( averaging winter and summer together) would be 4.5 – 5.5kWh requirement, and the couple in the Cape 2.5 – 3kWh.
Using these figures on a 150l geyser that consumes 7.67kWh to heat from cold( at 16 ° C) to hot( at 60 ° C), the solar system would need to be 65 – 35 % efficient— in SANS 6211-1 terms,‘ Q’ factors of 18 and 10 respectively( at 20MJ / m 2 per day).
Of course, homes being built are generally intended for more than just a couple, and the above figures are based on showering and exclude general hot water use. For four people, using a simplistic approach, you could just double the figures.
Potentially, that is where the problems really come in. Builders and developers want to install the cheapest solar thermal system they can find while still meeting( in theory anyway) the compliance requirements. So how can the building inspector act as the judge and the jury as to what is compliant when washing behaviour can be so different, and regional and seasonal factors come into play?
Into the next problem area. A tank needs to be SANS 151 approved, but does the solar system really need
Continued on page 30 >> www. plumbingafrica. co. za April 2017 Volume 23 I Number 2