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SWH vs heat pumps vs gas boilers vs solar PV

By James Green

Whichever way you look at it, water heating is extremely energy intensive.

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%.

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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.

Solar water heaters’ performance depends on the complete system.

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.

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.

Gas boilers enjoy the advantage of instant hot water, potentially saving on the heat loss of stored hot water, but the cost per litre of hot water is in theory very similar to an electrical resistance element, based on protection of Eskom’s prices and the minimum costs of gas as set by the Department of Energy and NERSA.

Heating oil boilers enjoy a cost advantage over gas, not because they are more efficient, but because paraffin does not suffer VAT and is subsidised by the government.

Solar photo voltaic (PV), where electricity is generated from solar panels, is not a suitable option for heating water using the existing electrical resistance elements in the geyser. Comparatively, the costs are up to five times higher than using solar water heating technology.

Based on South Africa’s high solar radiation, solar water heating should be the winner in terms of cost versus reward (depending on the system chosen).

However, a combination of hot water heating energy technologies may be the best solution, particularly on larger hot water requirements (commercial applications like hotels and hospitals). For example, solar is used to preheat and is then topped up by either gas, heating oil, or heat pumps. 

 James Green is the CEO of Ubersolar and Pay As You Go
 Solar, and a member of the SESSA Council.

Click below to read the March 2017 issue of Plumbing Africa

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