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How to reduce electricity dependence at home

An electrical engineer’s analysis.

With the rising electricity costs and declining power reliability, most South Africans are looking at alternatives.

Moneyweb asked electrical and electronic engineer Heino van Jaarsveldt, of DFR Engineers in Pretoria, what alternatives are available to smaller households and which are the most cost effective.

Van Jaarsveldt says the market is flooded with products promising substantial savings due to energy efficiency. It is however critical to first understand one’s household electricity consumption before determining where to reduce consumption.

He advises that one should first focus on the two main electricity consuming loads in a household: the stove and geyser. Here is his analysis.

Geyser

Electric water heaters

Most South African households make use of electrical tank-type water heaters to heat the water. A geyser blanket or an element timer are fairly cheap solutions to reduce standing losses.

Solar water heaters

Solar water heaters are commonly used to heat water for domestic applications.

  • Advantages: affordable, reliable and pollution-free (in operation) as they use renewable energy from the sun and don’t generate CO2 emissions in the water warming process.
  • Disadvantage: It is not available 24/7.

A solar water heater consists of a solar collector and a storage tank. Active solar water heaters include circulating pumps and controls. The energy from the sun is collected by means of the solar collector to heat water or another heat transfer fluid. This energy is then transferred directly or indirectly to the water being heated.

Heat pump

A domestic hot water heat pump is a very efficient water heating device, using a small amount of electricity to produce more heat energy. Unlike a solar geyser, a heat pump is not directly dependent on the sun and therefore can operate day and night.

The best way to describe the operation is to use the example of an air conditioner working in reverse. A heat pump extracts heat from the ambient air surrounding it, enabling it to heat the refrigerant which is also compressed to increase the temperature. The water is then heated by means of a heat exchanger. The electrical energy consumed by the heat pump is used to only run the fan and compressor of the system.

Case study

Geyser

For the purpose of the case study the energy consumption of an electric geyser, heat pump and solar water heater were compared in order to determine the estimated monthly savings. The calculations are based on a family of three with a hot water consumption of 75 litres per person per day.

The table below illustrates the estimated consumption and relevant energy cost for an electric geyser.

Electric geyser
Capacity (L) 200
Rating (kW) 4
Time (min) 156.75
Time (hours) 2.61
Energy consumption per day (kWh) 10.45
Energy consumption per month (kWh) 323.95
Energy tariff (Tshwane 2014) (R/kWh) R 1.47
Energy Costs (Per Month) R 476.21
   
Savings R 0.00

The table below illustrates the estimated consumption and relevant energy cost for a heat pump.

Heat pump
Capacity (L) 200
Rating (kW) 4
COP 3
Time (min) 156.75
Time (hours) 2.61
Energy consumption per day (kWh) 3.48
Energy consumption per month (kWh) 107.98
Energy tariff (Tshwane 2014) (R/kWh) R 1.47
Energy costs (per month) R 158.74
   
Savings R 317.47
   
Initial investment R 15 500.00

The graph below illustrates the estimated buyback period for a heat pump with an initial investment of R 15 500.00. The calculation of the buyback period is based on an escalation in energy tariffs of 12.69% for the first five years. From the graph shown below, it is clear that the estimated buyback period is approximately three and a half years. It is important to note that maintenance of the equipment and interest were not considered for the buyback period.

The table below illustrates the estimated consumption and relevant energy cost for a solar water heater.

Solar geyser
Solar collector size (square metre) 4
MJ/day per square metre 9.6
   
MJ/day 38.4
kWh 10.67
   
Available hot water 229.67
   
Energy costs (per month) R 0.00
   
Savings R 476.21
   
Initial investment R 20 507.46

The graph below illustrates the estimated buyback period for a solar water heater with an initial investment of R20 507.46. The calculation of the buyback period is based on an escalation in energy tariffs of 12.69% for the first five years.

From the graph below it is clear that the estimated buyback period is approximately three years. It is important to note that maintenance of the equipment and interest were not considered for the buyback period.

 

Stove

Most South African households make use of an electric stove for cooking purposes. The alternative is to use a gas stove which uses liquefied petroleum gas (LPG) as an energy source for cooking.

Induction cooking is also an alternative source cooking source. It heats a cooking vessel by means of electrical induction, instead of thermal conduction. Because electrical induction is used for the transfer of energy, the cooking vessel (pots and pans) must contain a ferromagnetic metal such as cast iron or stainless steel.

An induction cooker is faster and more energy efficient than a traditional electric cooking surface. Induction cookers allow for instant control of cooking power, similar to gas burners. It is estimated that the efficiency of an induction cooker results in 12% savings in energy for the same amount of heat transfer when compared with a traditional electric stove, but it still needs electricity. An induction cooker is not included in the case study.

Stove

For the purpose of the case study the energy consumption of an electric stove and a gas stove were compared in order to determine the estimated monthly savings. The calculations are based on a family of three, with the assumption that 3.25 kg of LPG is required per month for their cooking needs.

The table below illustrates the estimated consumption and relevant energy cost for an electric stove.

Electric stove
Energy rating (kW) 0.8
Hours 1.5
   
Energy consumption (kWh) per day 1.2
Energy consumption (kWh) per month 37.2
   
Energy tariff (Tshwane 2014) (R/kWh) R1.47
   
   
Energy costs (per month) R54.68

The table below illustrates the estimated consumption and relevant energy cost for a gas stove.

Gas stove
Gas consumption per hour (g) 60
Hours 1.5
   
Gas consumption per day (g) 90
Gas consumption per month (g) 2790
   
9 kg LPG cost R221.00
LPG cost per g R0.02
   
Energy cost (per month) R68.51

The tables show that the use of a gas stove for cooking will have no energy savings. This is due to the high price of bottled LPG. Reticulated LPG prices are lower than bottled LPG prices which may make the use of a gas stove more energy efficient.

It is also important to consider the price of LPG compared with that of electrical energy. The increase in electrical energy tariffs might make the use of gas for cooking a viable solution in the near future. If availability of energy is considered the overriding consideration, gas would be a better choice than electricity.

Conclusion

It is critical that the client determines the energy consumption of their equipment in order to accurately calculate the potential savings. The monthly energy consumption of equipment can easily be determined by means of an energy monitor.

*DFR Engineers is currently developing an energy efficiency package which will allow the user to identify the best energy efficient solutions specific to their application. The package will consist of a user manual which will discuss energy tariffs and energy consumption to the user in detail. An energy monitor will also be included in the package which will enable the user to log the energy consumption of various appliances within their homes. The final and most important piece of the equipment is a software model which will allow the user to input their data in order to identify energy saving solutions and to determine buyback periods for the selected options.

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