Heat Pump Simulation

Heating water is usually one of the largest components of your electricity bill.  One of the more efficient ways to heat water is using a heat pump.  We decided to try and get a sense of what savings can be expected in a real life case of a typical middle class home (consisting of two adults and two teenagers). The parents both showered every morning as well as shared a bath in the evenings.  While the  teenagers used a separate bathroom with its own geyser.  The teenagers preferred to bath in the evening.

We used the new PowerProphet consumption modelling tool to compare the expected electricity consumed to heat water using two conventional 200L (4kW) geysers, in comparison to heating the same amount of water using two 1.3kW heat pumps.

The assumptions used are listed at the bottom of the page.

Daily Values

The chart below shows the predicted half-hourly values (just like your meter would record them) for each scenario over a typical summers day.  The predicted energy consumption of the geysers is shown in blue, while that of the heat pumps is in red.

hp_khw_summerday

You can see that the heat pump is using much less electricity to heat the water in comparison to the geyser’s electric element.

The graph below shows the same thing for a typical winter’s day.

hp_khw_winterday

Because of the heat pump’s slightly lower COP in winter, the benefit gained isn’t quite as much as summer, but it still is substantial!

We used the accumulated energy graphs to understand how the consumption adds up over the course of each day.

hp_khwacc_summerday

The high COP of the heat pump in summer clearly dramatically reduces the electricity consumption heating water!

The graph below shows the same thing for a typical winter’s day:

hp_khwacc_winterday

Monthly Values

To get an idea of what sort of monthly savings one could expect we used the accumulated energy chart to plot the results for a typical summer month.  We also toggled the calculation to show monetary values instead of engineering values:

hp_khwacc_summerdmonth

As you can see, the model predicts a monthly cost of R797.94 to heat the water using conventional geyser’s, vs R205.90 using heat pumps.  A saving of R592.04 per month, or 74%, in summer.

The winter prediction is shown below:

hp_khwacc_winterdmonth

In this case the monthly cost of heating water using conventional geysers amounts to R982.08, vs R427.09 using heat pumps.  A saving of R554.99, or 57%, in winter.

If you approximate the annual expected savings by assuming three similar winter months and nine similar summer months then:

9 x R592.04 +3 x R554.99 = R6,993.33 (Remember this is for two heat pumps)

Of course that predicted annual saving is only as reliable as the assumptions its based on, but it does give you some idea of what amount to expect, more or less, in this particular home, given their daily consumption of hot water.

 Assumptions Used

  • The adults both showered in the mornings, one after the other, starting at 07:00 and then shared a bath at 21:00.  The teenagers each bathed, one after the other starting at 19:00 each evening.
  • In summer each shower lasts for 5 minutes.  But in winter people tend to shower for a bit longer, so we chose a time of 7.5 minutes.
  • The adult’s bath was a large corner bath (250L) which was normally filled to 75% capacity.  We assumed the bath used by the teenagers was larger than normal (200L), and which was half filled each time.
  •  Hot water for each bathroom is heated by a separate 200L geyser.  Both geysers use a 4kW heating element.  The set point temperature of both geysers is 60C.  We looked up the water heating times for each electric element in the hot water heating calculations on the PowerProphet site.  That was compared to a separate scenario in which the same water hot water consumption routine was modeled using 1.3kW heat pumps to heat the water.  In the same way we used the heat pump heating time calculations from PowerProphet.  Those calculations assume that the average COP‘s of the heat pumps used in the simulation varied from 3.9 in summer to 2.5 in winter.
  • The shower nozzle used was an efficient shower head flowing at 10L/min.
  • In all cases we assumed that the thermostat would reheat the water for ten minutes every four hours between main cold water heating events, to replace heat lost via the geyser insulation.
  • The PowerProphet water heating calculations assume that the cold water temperature in summer was 20C, while in winter it was 15C.
  • We used the current Ekurhuleni residential tariff of R1.18/kWh to estimate the monetary savings.

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