Using STCs to Understand Heat-Pump Hot-Water System Performance

by Richard Keech

2019-07-20

STCs are the basis for federal clean energy rebates, including for heat-pump hot water systems (HPHWSs) and solar-thermal hot-water systems.  However, they can also be informative in understanding the performance of HPHWSs.

Sanden - med res
Performance of heat-pump systems is indicated by number of STCs earned

(See also my earlier posts (here and here) on hot water.)

Background

There’s a strong need for hot-water systems to be as efficient as possible because they typically use a significant proportion of a home’s energy – often between 20% and 25%.  In Australia there is an important federal incentive scheme which rates hot-water systems’ efficiency and provides buyers with a rebate on the purchase price. It applies to systems which harvest renewable energy, i.e. solar-thermal hot-water systems and heat-pump hot-water systems.  This rewards buyers for choosing efficient systems by reducing their price in line with their performance – the more efficient the greater the rebate.

STCs

STCs. The instrument of  measurement of the scheme is the small-scale technology certificate (STC), just like with solar PV.   With hot-water, one STC corresponds to one megawatt hour (1 MWh) of avoided energy consumption over the system’s deemed life.  Systems are tested and awarded a number of STCs by the Clean Energy Regulator. The rebate earned for each system installed is in proportion to the number of STCs that the system earns.

STC deemed lifetime. The STCs correspond to performance over a deemed system life.  For solar-thermal hot water, this is 15 years.  For heat-pump hot water this is 10 years.

So, for HPHWSs, the number of STCs is energy saved, (sometimes referred to as ‘displaced energy’ or avoided consumption) and measured as MWh/decade.

STCs vs CoP.  The rating of systems in terms of STCs is of particular interest because it is probably the best proxy indicator of system performance. With heat-pump systems, many people talk about coefficient of performance (CoP) as a way of indicating system efficiency.  There are a number of problems with using CoP as a way of rating systems:

  • CoPs aren’t a standardised measure. In theory, CoP is a great way to assess a system for efficient performance.  The average CoP can be a good benchmark, but it’s not clear to me that there’s a broadly adopted industry standard for calculating this.  So two vendors could each cite a CoP value, but unless the basis for calculating the CoP is the same in each case, then you can’t have too much confidence in the comparison;
  • CoPs vary with temperature. It’s not terribly meaningful to say a system has a specific CoP value, because in reality CoP is a function of a number of factors, most notably ambient air temperature and the incoming water temperature.  For a hot-water heat pump, the higher the ambient air and water temperature, the higher the CoP.

So the benefit of using STCs for comparing performance is that:

  • The rating (STC count) is measured as part of a government-audited and carefully controlled framework which makes it a level playing field;
  • In practice, every type of heat-pump system on the market is registered for STCs even though it’s not mandatory, since without STCs a system cannot get the government rebate.

STC calculation. The manufacturer/importer of the system is responsible for having the system rated for STCs based on Australian standards.  The outcome is an estimate of the energy consumption in standard test conditions.  For a given system size (based on tank volume) there is a corresponding reference conventional system defined in the Australian Standard.  The number of STCs is then reckoned based on the difference between the estimated performance and the reference system performance as shown below:

S =  R – T

where:

S: STC count

R: Reference conventional system’s energy consumption (MWh over ten years)

T: Tested system’s estimated energy consumption (MWh over ten years)

Zoning. STC ratings are geographically based.  The standards recognise five climate zones across Australia, numbered 1 – 5, which are not the same as the eight numbered climate zones for Australian Homes.  So a system registered for STCs will be awarded a number of STCs based on its climate zone.   Systems don’t need to be registered for STCs in every zone.  However, if a system doesn’t have STCs for a given zone, then you can’t claim the rebate for installation within that geographic area.

Example Locales Zone
Darwin, Townsville 1
Alice Springs, Port Headland 2
Brisbane, Sydney, Adelaide, Perth 3
Melbourne, coastal Victoria 4
Canberra, Ballarat, Hobart 5

Table 1:  Heat-pump hot-water system climate zones

(Refer AS 4234 Figure A1, Table H9.1, Figure H9.1)

Zone 5. Climate Zone 5 corresponds to Alpine and other colder regions of Australia. This zone corresponds to building-code zones 7 and 8. Some systems don’t perform well in cold locales, so it’s not uncommon to see systems which don’t have an STC number for Zone 5.

Looking up STCs.  The Clean Energy Regulator (CER) publishes the official register of heat-pump systems here.  It is updated periodically.

Using STCs for efficiency comparison. Download the CER’s register spreadsheet and find the models you want to compare for the climate zone you’re located in.  The models with the highest STC number will have the best efficiency, so long as you’re comparing systems of the same size.  Remember the STC is based on a comparison (difference) with conventional reference systems of a similar size, so using STCs to compare systems of different size is meaningless.

Tank size scenario. Imagine your system of interest comes in three sizes, small, medium and large.  And the large-size system gets the highest number of STCs, and the small system the fewest.  This does not mean that the large system is more efficient than the small system.  The larger system will typically get more STCs simply by virtue that the conventional system it’s compared with uses more energy.

Reference systems. STC count doesn’t directly tell you how much energy a system uses in its test conditions.  For that you need to know how much energy the reference system uses and re-arrange the simple formula above, i.e.

T = R – S

Where the value of R is given below:

Size Reference energy consumption [MWh/decade]
Zone 1 Zone 2 Zone 3 Zone 4 Zone 5
Large 47.7 47.9 58.6 64.3 64.8
Medium 34.9 35.1 42.4 46.3 46.8
Small 21.2 21.4 25.8 28.6 28.9

Table 2: Reference system energy consumption

(Refer AS4234:2008 Table H9.3)

Examples

Let’s look at a few examples based on the medium-sized (250L)  Sanden Eco Plus.  Below is the data from the CER register for this unit.

Brand Model Eligible from Eligible to Certificates eligible based on zone
Zone 1 Zone 2 Zone 3 Zone 4 Zone 5
Sanden GAUS-250FQS 16 Nov 2018 31 Dec 2030 26 25 31 34 32

Table 3: Sanden system STCs in all climate zones

 

Example 1:  Inferred energy savings

Question: How much energy will using this system save when used in Melbourne?

Melbourne is Zone 4.  The system gets 34 STCs in Zone 4.  That’s 34 MWh/decade saved, which is 9.3 kWh/day.

Tip.  Divide the STC value by 3.65 to get kWh/day saved.

 

Example 2:  Inferred energy use

Question: How much energy will the system use in Sydney?

Sydney is Zone 3. The system gets 31 STCs in Zone 3.

S: 31  (STCs, from the CER’s spreadsheet)

R: 42.4 (MWh/decade, from the table above – Zone 4, Medium)

Inferred consumption (T=R-S):   11.4 MWh/decade = 3.1kWh/day

Note that the test scenarios is based on a nominal ‘load’ measured in MJ/day of thermal energy delivered in the water (AS4234 Table H9.3).  Converting this to L/day of hot water delivered isn’t straightforward because of the number of variables involved.  Load varies on climate zone and tank size.

 

Example 3:  Comparing different size tanks on the same system

Question: How much more energy will the same system use with the larger tank?

From Example 2, The medium-size system uses 3.1kWh/day (in Sydney).

Large model: GAUS-315FQS: STCs: 32 (from CER’s spreadsheet)

R = 58.6 (Table 2, Zone 3, Large)

Inferred consumption (T=R-S):  T = 58.6 – 32 = 26.6 MWh/decade = 7.3kWh/day

Comparison of energy use:

  • Medium: 3.1kWh/day  (from Example 2, above)
  • Large: 7.3kWh/day

Note that this doesn’t mean that, for the same usage, the large system will use 7.3 and the medium will use 3.1 because the testing of the large system assumes 50% greater delivered hot-water thermal energy than the medium system.

If we assume that the mains energy is in direct proportion to the delivered thermal energy, then the medium system will use 1.5 x 3.1 =4.65kWh/day to do the same work as the large system (but with the risk that the medium system might sometimes not quite keep up with demand).  So using the large tank uses approximately 7.3-4.65=2.65kWh/day more mains energy, but without any assurance that the water will always be a sufficient temperature.

 

 

 

 

 

 

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