African energy
15
SWH vs heat pumps vs gas boilers vs solar PV
Whichever way you look at it , water heating is extremely energy intensive .
By
James Green
In South Africa with around seven million electric geysers in homes , approximately 18 % of Eskom ’ s power is used in heating water . Add in the hot water in business and commercial use , and the figure jumps to as high as 25 %.
For the homeowner , water heating can constitute as much as 40 – 60 % of their monthly electricity account , depending on washing behaviour .
Electrical resistance elements as used in geysers are cheap to produce and use basic technology . Ranging in size from 2kW to 5kW for various tank sizes , the size of the element — not the amount of electricity consumed — determines how fast the water is heated . For example , a 150l geyser will use 7.6kWh to heat from cold to hot , and with a 3kW element will take about 2.5 hours to heat ( 7.6kWh ÷ 3kW element = 2.5 hours ).
Solar water heaters ’ ( SWH ) performance depends on complete system performance , not just the size or type of solar collector . Different types and configurations , split , integrated , direct , indirect , thermos-syphon , forced circulation , flat plate , evacuated tube , collector size , and special coatings all affect the end performance result in hot water generated and stored , which in turn can be converted back to kilowatt-hour saved .
All SWHs have the amount of time required to generate the kilowatt-hour output in common , which will be six to seven hours in winter and six to eight hours in summer . Consequently , higher performance output can generally be expected in summer .
Domestic heat pumps — depending on size and configuration — should ( in theory anyway ) save approximately 65 % of the electricity used in heating water by an electrical resistance element . The time taken to heat the water will depend on the real achieved output from the heat pump , and the efficiencies will typically be greater the higher the ambient air temperature .
Solar water heaters ’ performance depends on the complete system .
Gas boilers running on piped gas or LPG performance will depend on the BTU output . Hot water can be delivered instantaneously to the tap , or stored in a water tank and topped up . Heating oil boilers running on kerosene ( paraffin ) share similar characteristics to gas boilers .
From a cost perspective , the greatest energy savings should be achieved with solar , depending on the output of the system . Heat pumps can become a contentious area , as many heat pumps in the domestic sector fail to achieve any real electrical savings in winter over an electrical resistance element . Larger heat pumps , as used for heating large volumes of water ( for example in a hotel ), and where solar may not be suitable due to space limitations , can prove to be a suitable option .
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Happybaby . co . za www . plumbingafrica . co . za March 2017 Volume 23 I Number 1