Mixergy/004/VR/NPL

VERIFICATION REPORT Mixergy/004/VR/NPL

Mixergy Tank

Prepared By: National Physical Laboratory Prepared for proposer: Mixergy Limited Status: Final Date: 8 December 2016

Mixergy/004/VR/NPL

Date signed: 8 December 2016

London, United Kingdom

Mixergy/004/VR/NPL

TABLE OF CONTENTS 1 INTRODUCTION ............................................................................................................................. 2

1.1 Name of technology ............................................................................................................ 2

1.2 Name and contact of proposer .......................................................................................... 2

1.3 Name of Verification Body and responsible of verification ........................................ 2

1.4 Organisation of verification including experts, and verification process ............... 2

1.5 Verification Process ............................................................................................................ 3

1.6 Deviations from the verification protocol ...................................................................... 3

2 DESCRIPTION OF THE TECHNOLOGY ......................................................................................... 4

2.1 Summary of description of the technology .................................................................... 4

2.2 Intended application (matrix, purpose, technologies and technical conditions) .. 5

2.2.1 Matrix ................................................................................................................................ 5

2.2.2 Purpose ............................................................................................................................. 6

2.3 Associated environmental emissions and/or impacts ................................................ 6

2.4 Verification parameters definition .................................................................................. 6

2.4.1 Performance parameters .............................................................................................. 6

2.4.2 Operational parameters ................................................................................................ 7

2.4.3 Environmental parameters ........................................................................................... 7

3 EXISTING DATA ............................................................................................................................. 7

3.1 Accepted existing data ....................................................................................................... 7

4 EVALUATION ................................................................................................................................. 8

4.1 Calculation of performance parameters ........................................................................ 8

4.1.1 Increase in useable hot water yield for Mixergy tank c.f. standard tank when

both heated to the same initial temperature ........................................................................ 8

4.1.2 Volume of useable hot water delivered from 75 litre Mixergy tank ..................... 8

4.2 Evaluation of test quality ................................................................................................... 8

4.2.1 Control data ...................................................................................................................... 8

4.2.2 Data Management ......................................................................................................... 14

4.2.3 Record Keeping ............................................................................................................. 14

4.2.4 Audits ............................................................................................................................... 14

4.2.5 Deviations ....................................................................................................................... 14

4.3 Verification results (verified performance claim) ..................................................... 14

4.3.1 Performance parameters ............................................................................................ 14

4.3.2 Operational parameters .............................................................................................. 16

4.3.3 Environmental parameters ......................................................................................... 16

4.4 Recommendations for the Statement of Verification ................................................ 17

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5 QUALITY ASSURANCE ................................................................................................................. 17

6 REFERENCES ............................................................................................................................... 18

List of Figures Figure 1: Variation of hot water demand, electricity consumption and timing of peak versus off peak

hours ....................................................................................................................................................... 4

Figure 2: Variation of UK electricity carbon intensity throughout the year and throughout particular

days in the year ....................................................................................................................................... 5

Figure 3: Image of the Mixergy tank ....................................................................................................... 5

Figure 4: (Left) Platinum reference thermometer and (right) calibration bath used to provide

reference temperature ........................................................................................................................... 9

Figure 5: Calibrated electronic scales purchased for the purposes of the test ...................................... 9

Figure 6: The changing weight of the scales over time during three independent draw tests

conducted at 15 l/min ........................................................................................................................... 10

Figure 7: Reference timing device used to deduce timing uncertainty ................................................ 11

Figure 8: Energy logger ......................................................................................................................... 11

Figure 9: Sensitivity of useable volume calculations as a function of measurement time step. .......... 13

Figure 10: Useable volume per litres installed final results .................................................................. 15

Figure 11: Ambient temperatures recorded during each test .............................................................. 16

Table 1: Overview of the verification process under the EU ETV Pilot Programme ............................... 3

Table 2: List of performance parameters and their values ..................................................................... 7

Table 3: List of operational parameters and their values ....................................................................... 7

Table 4: Sensitivity of results to numerical method (Mixergy tank) ..................................................... 12

Table 5: Useable volume results delivered for Mixergy tank ............................................................... 15

Table 6: Useable volume results delivered for standard tank .............................................................. 15

Table 7: Energy logged prior to test (kWh) results to within +/- 0.015 kWh ........................................ 16

Table 8: Peak temperature of base of cylinder (°C) prior to test (+/- 1 °C) .......................................... 16

Table 9: Breakdown of responsibility between the organisations involved with the verification ....... 17

Equation 1: Calculation of the useable volume of hot water delivered from the hot water tank ......... 8

Equation 2: Calculation of the normalised useable mass of hot water delivered from the hot water

tank normalised to the tank’s capacity ................................................................................................... 8

Equation 3: Development of worst case error bars for useable volume measurement ...................... 12

Equation 4: Heat losses associated with a hot water tank ................................................................... 13

Appendix 1: Quick scan …………………………………………………………………………………………………………………..19 Appendix 2: Proposal ……………………………………………………………………………………………………………………...30 Appendix 3: Specific verification protocol ……………………………………………………………………………………….40 Appendix 4: Test report …………………………………………………………………………………………………………………..60 Appendix 5: Audit report …………………………………………………………………………………………………………………83

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1 INTRODUCTION This report compiles the relevant documents included in the verification process for the Mixergy Tank,

which has been carried out under the framework of the European Union (EU) Environmental

Technology Verification (EU-ETV) pilot programme.

1.1 Name of technology

The Mixergy Tank is a hot water tank that makes use of a novel inlet diffuser and angled heating

element scheme to increase the utilisation of stored water within a domestic hot water tank, created

by Mixergy Limited.

1.2 Name and contact of proposer

Name: Peter Armstrong Address: Mixergy Limited

2 Canal View Wharf Farm Eynsham Road Cassington Oxfordshire OX29 4DB UK

Tel: +44 7830 840 311 Email: peter.armstrong@mixergy.co.uk

1.3 Name of Verification Body and responsible of verification

Name: National Physical Laboratory Address: Hampton Road

Teddington Middlesex TW11 0LW UK

Tel: +44 20 8943 6964 Verification Expert: Paul Miller Verification Expert: Emma Richardson Email: paul.miller@npl.co.uk Email: emma.richardson@npl.co.uk

1.4 Organisation of verification including experts, and verification process

The verification was coordinated and managed by NPL, which has been accredited to the requirements

of ISO/IEC 17020:2012 for an inspection body type A. Tests were undertaken at Newark Copper

Cylinders Ltd by Peter Armstrong (Mixergy Ltd), Ren Kang (Mixergy Ltd) and Mark Smith (Newark

Copper Cylinders Ltd).

Internal and external technical experts were assigned to provide an independent review of the

planning, conducting and reporting of the verification process.

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Internal Technical Expert: Dave Lowe, Senior Research Scientist, Temperature & Humidity, NPL, e-mail: dave.lowe@npl.co.uk

External Technical Expert: Marc Zanchetta, Consultant, GasDynamics Ltd, e-mail: mzanchetta@gasdynamics.co.uk

1.5 Verification Process

Stage Responsibility Document

Preliminary phase Verification Body

Quick Scan

Contract/Proposal

Specific Verification Protocol

Testing phase Test Body Test Plan

Test Report

Verification phase Verification Body Verification Report

Statement of Verification

Table 1: Overview of the verification process under the EU ETV Pilot Programme

Quality assurance was undertaken by all technical experts involved in the verification process (see

section 5). The Statement of Verification will be issued by NPL following a two week review period of

the Verification Report. The Statement will be registered by the European Commission and published

on their website at the following link: http://iet.jrc.ec.europa.eu/etv/

1.6 Deviations from the verification protocol

Specific Verification Protocol (SVP) 3.2 (please see Appendix 3) states the header tank was

held at 20 °C ± 2 °C rather than 18.0 °C ± 0.5 °C (please see Appendix 4: Test Report 3.4 (point

2) and Table 3 below).

SVP 4.1 states the room temperature where the tests would be carried out would be 20 °C ±

2 °C. The actual temperature during testing was 19 °C ± 3 °C.

SVP 4.3 states the data management would meet the requirements of ISO17025. The audit

identified that “Evidence of testing of both the data acquisition software and the data analysis

software is required (Section 5.4.7 of EN ISO/IEC 17025:2005)”.

SVP 4.4.1 states that the top temperature for calibration of the sensors (type-T

thermocouples) would be 80 °C, but was 60 °C (Test Report A2.1.2).

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2 DESCRIPTION OF THE TECHNOLOGY

2.1 Summary of description of the technology

The Mixergy tank makes use of a novel inlet diffuser and angled heating element scheme to increase

the utilisation of stored water within a domestic hot water tank. The inlet arrangement reduces mixing

to increase the amount of hot water that can be recovered from a given pre-heated tank volume.

An indirect benefit, not verified by this work package, is that this performance enhancement enables

the tank to make better use of off-peak tariffs, for instance during the night for an Economy 7 tariff.

The objective of an electric hot water tank in many power systems is to heat up during times when

energy is cheap and is being produced at a low carbon intensity. For instance, in the UK, the average

carbon intensity of electricity production is in the range of 10 % to 28 % lower during off-peak hours.

Consequently, systems are configured to automatically turn on during off-peak periods to exploit low

carbon intensity and low electricity prices.

However, if the tank runs out of hot water during peak periods, electricity must be consumed when

the carbon intensity is high, leading to excessive carbon emissions. Figure 1 illustrates the typical

demand pattern for hot water throughout the day against UK electricity demand with annotations

showing peak and off peak hours, whilst Figure 2 illustrates the variation in carbon intensity for

available data during 2015. Figure 3 shows a transparent view of the Mixergy tank.

Figure 1: Variation of hot water demand, electricity consumption and timing of peak versus off peak hours

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2.2 Intended application (matrix, purpose, technologies and technical

conditions)

The intended application of the technology for verification is defined in terms of the matrix and the

purpose.

2.2.1 Matrix

Potable hot water stored within stainless steel tanks.

Figure 2: Variation of UK electricity carbon intensity throughout the year and throughout particular days in the year

Figure 3: Image of the Mixergy tank

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2.2.2 Purpose

The purpose of the technology is to deliver more useful hot water per unit of pre-heated installed volume. The hot water from the tank is used for domestic applications such as bathing, showering and cleaning. To determine whether the technology has fulfilled this purpose, the volume of hot water delivered over a range of flow rates was measured and compared to a standard tank design with equivalent volume. Here, standard tank means one conforming to British Standard BS 699:1984. Prior to testing, both the Mixergy and standard tanks were heated to the same temperature set point associated with their respective thermostats.

2.3 Associated environmental emissions and/or impacts

Compared to a standard 75 litre hot water tank, the Mixergy system claims to deliver 15 % to 30 %

more useable hot water from the same installed volume, heated to an initial temperature of 60 °C, at

a minimum threshold delivery temperature of 43 °C. The improved performance enables several

potential benefits:

When using a standard 75 litre hot water tank, the Mixergy design is able to deliver more

useable hot water from a fully heated tank. This reduces the requirement for additional

heating of the water to maintain a useable temperature.

Alternatively, by delivering more hot water than a standard configuration, a smaller tank

volume can be specified for a given application leading to a reduction in standing heat losses.

In addition, off-peak energy can be used to deliver more hot water with a reduced risk that

energy will be required during peak times to boost the temperature. Within the UK, there are

several examples of this type of time-use tariff e.g. overnight, for instance schemes referred

to as Economy 7 or Economy 10.

2.4 Verification parameters definition

This section will examine the different verification parameters of the technology.

2.4.1 Performance parameters

The amount of hot water is defined by considering the water volume that could be delivered from the tank outlet at 43 °C when mixed with cold mains water at the same temperature as the tank’s inlet. This mixed temperature was chosen since it reflects the maximum setting associated with common thermostatic mixing valves [1] and is commonly accepted as a safe value for useable hot water for domestic purposes. The following performance claims were made in the SVP (Table 2 and Table 3):

A. An increase in useable hot water yield per unit installed tank volume up to 30 % across flow

rates of 2 to 15 l/min, from a 75 litre tank.

B. The 75 litre Mixergy test tank can deliver up to 120 litres of useable hot water compared to

90 litres of useable hot water from a standard 75 litre tank (equivalent to an increase of circa

33 %).

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The Mixergy tank is able to deliver more litres of useable hot water per unit installed volume compared

to a conventional tank, designed according to British Standards, where the useable temperature is

43 °C.

2.4.2 Operational parameters

Operational parameters relate to the technical conditions of the intended application (Table 3):

2.4.3 Environmental parameters

Other than the performance parameters, no additional environmental parameters will be measured.

3 EXISTING DATA

3.1 Accepted existing data

No existing data has been accepted for the verification of the Mixergy tank. However, note that

because of the iterative nature of the SVP, test measurements were carried out before the final agreed

version of the SVP and before the formal test plan was submitted to NPL.

1 The tank temperature is measured above the immersion heater, where convective mixing means the water is at uniform temperature, and a thermometer will read a temperature representative of the immersion heater thermostat setting [3].

PARAMETER CLAIMED VALUE

Increase in useable hot water yield for Mixergy

tank c.f. standard tank when both heated to

the same initial temperature

15 % > < 30 %

Volume of useable hot water delivered from

75 litre Mixergy tank

120 litres (at 15 l/min)

Table 2: List of performance parameters and their values

PARAMETER VALUE

Domestic water tank volume 75 litres

Cold water input 20 °C +/- 2 °C

Heated water (immersion heater) 60 °C +/- 0.5 °C

Threshold temperature range 43 °C

Flow rate range 2 to 15 l / min

Energy required to heat test tanks to 60 °C 1 3.4 kWh

Table 3: List of operational parameters and their values

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4 EVALUATION

4.1 Calculation of performance parameters

On completion of the test, the following calculation was performed:

𝑉�� = �� ∫ (1 +𝑇𝑜𝑢𝑡 − 𝑇𝑢

𝑇𝑢 − 𝑇𝑖𝑛)

𝑡𝑠

0

𝑑𝑡

Equation 1: Calculation of the useable volume of hot water delivered from the hot water tank

Equation 1 computes the amount of useful hot water, 𝑉��, that would be delivered at a threshold

temperature, 𝑇𝑢. 𝑇𝑢 is assumed to be 43 °C which reflects the sort of temperature a user would

experience during a bath or shower. 𝑉�� depends on how the outlet temperature 𝑇𝑜𝑢𝑡 and inlet

temperature 𝑇𝑖𝑛 changes up to the point where the integral time limit, 𝑡𝑠 is encountered. The limit 𝑡𝑠

occurs where 𝑇𝑜𝑢𝑡 drops below 𝑇𝑢. Throughout the test, the water flow rate was held constant at a

rate of ��.

4.1.1 Increase in useable hot water yield for Mixergy tank c.f. standard tank

when both heated to the same initial temperature

Equation 1 was evaluated for both tanks, and the difference taken.

4.1.2 Volume of useable hot water delivered from 75 litre Mixergy tank

Equation 1 was evaluated for the Mixergy tank.

4.2 Evaluation of test quality

4.2.1 Control data

This section details all of the measurement parameters and the steps taken to quantify

errors/uncertainties.

In addition to the repeatability of the measurements, an estimate of the overall uncertainty given

known and estimated measurement errors was required. To account for these errors, first the

equation used to determine the final verified performance parameter, useable mass, needs to be

considered:

𝑀𝑢 =��

𝑀𝑇∫ (1 +

𝑇𝑜𝑢𝑡 − 𝑇𝑢

𝑇𝑢 − 𝑇𝑖𝑛)

𝑡𝑠

0

𝑑𝑡

Equation 2: Calculation of the normalised useable mass of hot water delivered from the hot water tank normalised to the tank’s capacity

Equation 2 computes the amount of useful hot water, 𝑀𝑢, that would be delivered at a threshold

temperature, 𝑇𝑢. Typically 𝑇𝑢 would be between 40 °C to 45 °C. In this case, a temperature of 43 °C

was selected for 𝑇𝑢 since this is the upper temperature limit associated with the output from Bristan™

thermostatic mixing valves [1]. The yield of useable mass, 𝑀𝑢, depends on how the outlet temperature

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𝑇𝑜𝑢𝑡 and inlet temperature 𝑇𝑖𝑛 changes up to the point where the integral time limit, 𝑡𝑠 is

encountered. The limit 𝑡𝑠 occurs where 𝑇𝑜𝑢𝑡 drops below 𝑇𝑢. Throughout the test, the water flow rate

was held constant at a rate of ��. The capacity of the tank, around which the results are normalised,

is expressed as 𝑀𝑇.

To understand how measurement errors associated with all of the above parameters affects the

overall uncertainty around 𝑀𝑢, each parameter was considered in turn:

Temperature

In order to ensure that all measurement uncertainties are properly accounted for, the temperature

sensors were checked against a newly bought platinum resistance thermometer which comes with a

5-point calibration between -200 °C and +200 °C from a UKAS accredited laboratory. The sensors are

T-Type thermocouples with an uncertainty of ± 0.5 °C. The readings from the test sensors and

reference thermometer were immersed within stirred water bath which was initially filled with a

mixture of ice and water before being gradually heated to 80 °C. The output of the thermometers

were compared at temperatures of 0 °C, 20 °C, 40 °C and 60 °C. The calibration equipment is shown

in Figure 4.

It was found that all sensors were within 1 °C of the reference thermometers measurement. An

uncertainty of 1 °C is assumed for all measurements for the purposes of calculation. Mixergy’s analysis

does not assume errors are uncorrelated and therefore they did not take them in quadrature, this is

a conservative assumption which implies that temperatures are potentially correlated due to common

instrumentation.

Flow Rate Error

The flow rate measurement was performed by recording the changing weight on a set of electronic

scales underneath the test sump. The scales, shown in Figure 5, were bought new with a calibration

certificate produced to standard ISO 9001:2008 with a quoted accuracy of ± 0.1 kg.

Figure 5: Calibrated electronic scales purchased for the purposes of the test

Figure 4: (Left) Platinum reference thermometer and (right) calibration bath used to provide reference temperature

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The flow rate measurement process involved computing the gradient of the electronic scale’s output

over time. This is illustrated by Figure 6, where the raw weight data is plotted with a linear curve fit

and associated equation for three independent draw tests conducted at 15 l/min.

Figure 6: The changing weight of the scales over time during three independent draw tests conducted at 15 l/min

The raw data in Figure 6 was sampled at 100 ms intervals and has had linear functions regressed to it

to determine the slope and linearity using the ‘R² test’. As can be seen, the fit is very close and the

gradient (in kg/sec) is to within 3 decimal places for each test. Given that the timing error (see next

section) is negligible, it is assumed that the flow rate error measurement is entirely down to the

resolution of the scales (± 0.1 kg). During each test, the scales were logged as they registered a change

in mass of between approximately 20 kg and 100 kg. Therefore, a worst case range of between 19.9 kg

and 100.1 kg, and 20.1 kg and 99.9 kg; or a mass of 79.8 kg to 80.2 kg was assumed. At 15 l/min, this

mass would be delivered in approximately 320 seconds leading to a flow rate of (0.25 ±

0.000625) kg/sec. Throughout the tests, Mixergy assumed a flow rate uncertainty of ± 0.001 kg/sec as

a conservative estimate.

Timing Error

The flow rate uncertainty assumed that the timing errors were negligible, this was verified using a

digital time reference (Figure 7) which was bought new and calibrated according to ISO17025 by a

UKAS accredited facility. The timer was triggered manually against software time recordings to verify

that any discrepancy with true time was negligible.

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Energy Logging Error

The current to the immersion elements was fed through an energy logger (Figure 8). The meters are

certified to be within ± 0.5 % of the rating of the element and have been independently verified by

SGS Ltd under EC directive 2004/22/EC.

Treatment of the equations

In order to account for the uncertainty associated with the performance claim, the measurement

errors associated with all test parameters had to be accounted for. For instance temperature

measurement errors are often assumed to be uncorrelated and therefore taken in quadrature [2]. In

this case, the test rig made use of thermocouple junctions which are referenced to a single junction.

There is a chance that errors are therefore correlated and so a more conservative approach to the

analysis has been taken. In this instance, Mixergy have considered the worst errors associated with

each parameter and explicitly computed the results with them included, such that the extremes of the

resulting error bounds are computed. For Equation 2 this works out as follows, resulting in Equation

3:

Figure 7: Reference timing device used to deduce timing uncertainty

Figure 8: Energy logger

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𝑀𝑢 =𝑚

𝑀𝑇

∫ (1 +

𝑇𝑜𝑢𝑡 − 𝑇𝑢

𝑇𝑢 − 𝑇𝑖𝑛)

𝑡𝑠

0

𝑑𝑡

+𝐸𝑟𝑟(𝑀𝑢) = 𝑚

𝑀𝑇 − ∆𝑀𝑇

∫ (�� + ∆��) [1 +

𝑇𝑜𝑢𝑡(𝑡) + ∆𝑇𝑜𝑢𝑡 − 𝑇𝑢

𝑇𝑢 − 𝑇𝑖𝑛 + ∆𝑇𝑖𝑛]

𝑡𝑠

0

𝑑𝑡

−𝐸𝑟𝑟(𝑀𝑢) = 𝑚

𝑀𝑇 + ∆𝑀𝑇

∫ (�� − ∆��) [1 +

𝑇𝑜𝑢𝑡(𝑡) − ∆𝑇𝑜𝑢𝑡 − 𝑇𝑢

𝑇𝑢 − 𝑇𝑖𝑛 − ∆𝑇𝑖𝑛]

𝑡𝑠

0

𝑑𝑡

Equation 3: Development of worst case error bars for useable volume measurement

Numerical sensitivity

Equation 1 is computed as a continuously varying integral, whereas in practice the solution was arrived

at by summing the discrete samples associated with the timing interval selected for the duration of

the test. This introduces two sources of error. Firstly, there is the error associated with the method

used to interpolate between sample points. It could be assumed that the measured values are

constant between samples and perform the integral as a sum (referred to here as the summed

method). Alternatively, the trapezoidal method, which assumes a linear interpolation between sample

points, could be used. Table 4 shows the computed values for useable volume and the associated

discrepancy between using the summed and trapezoidal methods. The difference between the

methods was less than 0.1 %. Given the small disparity between the results it was decided not to

investigate more elaborate numerical methods such as the cubic spline or other regressed analytical

functions.

Nominal Flow Rate

Useable Mass (kg/kg) Summed Method

Useable Mass (kg/kg) Trapezoidal Method

Discrepancy (%)

2.00 1.5636 1.5634 0.01

8.50 1.5643 1.5635 0.05

15.00 1.6072 1.6060 0.07 Table 4: Sensitivity of results to numerical method (Mixergy tank)

In addition to the numerical method, the sample time also had an influence on the result. To account

for this, the following procedure was undertaken:

1. Tabulate data such that all parameters begin at the initiation point of the draw cycle test;

2. Compute integrals associated with Equations 1 and 2 via a trapezoidal numerical integration

between sample points;

3. Record result and repeat steps 1 and 2 at successively lower timing resolutions so that time-

step error can be evaluated and calibrated out. In other words, the underlying sample rate is

0.5 seconds, the numerical integration will be performed for all samples first before the

process is repeated for sample rates of 1 second and 1.5 second the change in result that

arises is attributable to the time step error. The results are plotted and extrapolated to a

sample time step → 0 seconds to account for the time step error.

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This procedure was applied to a test conducted on the standard tank during a draw test at a target

flow rate of 15 l/min.

Figure 9: Sensitivity of useable volume calculations as a function of measurement time step

Influence of time step on calculated result

Extrapolation as illustrated in Figure 9 suggests that the difference in result between a sample of

1 second and 0 seconds is 88.4 kg compared to 89.7 kg which equates to a sample error of 1.5 %. In

the interest of trading uncertainty against the volume of data that had to be managed, it was decided

that a sample of 1 second would be used for all measurements. All error bounds were increased in

size by 2 % to provide a conservative penalty associated with both time step and numerical method

errors.

Influence of environmental conditions

During the test, the ambient temperature varied between 16 °C and 19 °C. To quantify how significant

this factor is, we consider that heat losses can be expressed according to the following equation,

where 𝑄𝑙 are the heat losses in watts (W), ℎ is the heat transfer coefficient associated with the

insulation, A is the surface area of the tank, 𝑇ℎ is the temperature of the water stored within the tank

and 𝑇𝑎 is the ambient air temperature:

𝑄𝑙 = ℎ𝐴(𝑇ℎ − 𝑇𝑎)

Equation 4: Heat losses associated with a hot water tank

The tanks have been designed to ensure heat losses are no greater than 2 kWh/day (𝑄𝑙 = 83 W). This

corresponds to an approximate value for ℎ𝐴 of 2.1 W/°C. For the hour during which the test tank is

standing after heating, 𝑄𝑙 would be between 85 W and 92 W at temperatures of 19 °C and 16 °C

respectively. The state of charge of the tank between an initial temperature of 18 °C and heated

temperature of 60 °C is 3.68 kWh. After 1 hour, the state of charge would have reduced to 3.5833

kWh and 3.5895 kWh respectively, a difference of less than 0.2 %.

1

1.05

1.1

1.15

1.2

1.25

1.3

0.5 1 1.5

Use

able

mas

s p

er u

nit

inst

alle

d m

ass

Time-step (seconds)

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Summary of stated uncertainty

In summary, the reported uncertainties are derived via explicit computation of the results assuming

worst case correlated errors, as illustrated by Equation 3. Repetition of the experiments found that

results could be consistently reproduced whilst a time step of 1 second was considered to be adequate

along with the use of a trapezoidal integration method.

4.2.2 Data Management

All data was compiled from the host PC into a dated spreadsheet on completion of each test. Data was

hosted on a shared folder system to ensure multiple back-ups were distributed across personnel

computers.

4.2.3 Record Keeping

Records of tests conducted are kept in the test room laboratory.

4.2.4 Audits

An audit was carried out by Helen McEvoy (Senior Research Scientist, Temperature and Humidity

Standards, NPL) on 17 August 2016. The outcome of that audit can be seen in Appendix 5 (Audit

report).

4.2.5 Deviations

Please see section 1.6.

4.3 Verification results (verified performance claim)

The results described below have been audited. The audit reported:

In general the test facility, test methodology, data analysis process and staff training/

competence are fit for purpose and meet those requirements of ISO17025 which are

applicable for this test process.

The test procedure and the test results obtained to date, and which are presented in the Test

Report, are sufficient to confirm the validity of the methodology and the ‘proof of concept’ of

the Mixergy design of tank.

4.3.1 Performance parameters

The Mixergy tank was able to deliver between 7.2 % and 31.1 % more useable hot water per installed

volume of water across flow rates of 1.9 l/min to 15 l/min as compared to the claimed values in Table

2. This is illustrated in Figure 10 which shows the measured values which are annotated with their

associated error bounds. The derivation of the error bounds are discussed in 4.2 and Appendix A2.1.2

of the Test Report.

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Figure 10: Useable volume per litres installed final results

Table 5 and Table 6 show the data plotted in Figure 10 for the Mixergy and standard tanks respectively.

NOTE that results are in kg of useable hot water per kg of installed water. These figures can be

interchanged with litres by assuming 1 litre ≈ 1 kg. This is true to within 0.5 % for typical atmospheric

pressures and temperatures which prevailed during test conditions. Uncertainties are reported at

coverage factor k = 1.

Mixergy Useable Volume

Flow Rate

(kg/sec)

Flow Rate (kg/min

(approx. lpm))

(-)Err Mu (kg/kg) (litre/litre approx.)

(+)Err Percentage increase

within error margin (%)

Delivered volume

from 75 litre tank

assuming 1 kg/litre

0.031 1.87 0.07 1.56 0.08 7.2 117 ± 5

0.139 8.35 0.04 1.56 0.05 13.4 117 ± 3

0.249 14.96 0.05 1.61 0.04 31.1 121 ± 4

Table 5: Useable volume results delivered for Mixergy tank

Standard Useable Volume

Flow Rate (kg/sec)

Flow Rate (kg/min (approx. lpm))

(-)Err Mu (kg/kg) (litre/litre approx.)

(+)Err Delivered volume from 75 litre tank

assuming 1 kg/litre

0.030 1.82 0.04 1.35 0.04 101 ± 3

0.137 8.20 0.03 1.30 0.04 98 ± 2

0.250 14.99 0.03 1.16 0.03 87 ± 2

Table 6: Useable volume results delivered for standard tank

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 2.5 5 7.5 10 12.5 15

Use

able

ho

t w

ater

mas

s p

er in

stal

led

m

ass

(kg/

kg)

Flow Rate (kg/min (approx litres per minute))

Mixergy Tank

Standard Tank

Mixergy/004/VR/NPL

16

4.3.2 Operational parameters

Table 7 details the energy consumed by the immersion elements during each of the tests.

Target Flow Rate

2 lpm 8.5 lpm 15 lpm

Tank Mixergy Tank 3.38 3.36 3.40

Newark 3.14 3.19 3.12

Table 7: Energy logged prior to test (kWh) results to within +/- 0.015 kWh

During the tests, the base temperature of the hot water cylinder was logged during operation. The

Mixergy tank attained a higher temperature at the base due to orientation of the heating element as

evident in Table 8:

Target Flow Rate

2 lpm 8.5 lpm 15 lpm

Tank Mixergy Tank 56.20 56.30 56.50

Newark 22.70 22.90 22.00

Table 8: Peak temperature of base of cylinder (°C) prior to test (+/- 1 °C)

4.3.3 Environmental parameters

The ambient temperature during each test run is shown in Figure 11.

Figure 11: Ambient temperatures recorded during each test

Mixergy/004/VR/NPL

17

4.4 Recommendations for the Statement of Verification

The Mixergy tank with a control thermometer installed as described can deliver more hot water for a

75 litre installed volume, at the level of up to 30 % given in the performance claim. Some of this is due

to more initial heat input, and at low flow rate all the benefit is from increased storage density. At

higher flow rates there is additional benefit from the reduction of mixing. The Mixergy tank with a

volume of 75 litres delivered 120 litres of hot water at the threshold 43 °C compared to 90 litres

delivered by a tank built to British Standards.

Therefore, NPL deem the deviations from the SVP outlined in section 1.6 to be acceptable and the

claims 2.4.1(A) and 2.4.1(B) that were made in the SVP have been met.

5 QUALITY ASSURANCE Table 9 demonstrates the breakdown of review between the organisations involved with the

verification.

The verification body, led by Paul Miller, reviewed and approved the test plans and test reports from

the test houses.

During the verification process the Proposer, represented by Peter Armstrong, had the following tasks:

Review the specific verification protocol

Review and approve the test plan and test report, then submit to verification body

Review the verification report

Accept the Statement of Verification

The specific verification protocol and the verification report required external review according to the

EU ETV General Verification Protocol (European Commission, 2016). The external review was

undertaken by Marc Zanchetta (GasDynamics Ltd, e-mail: mzanchetta@gasdynamics.co.uk).

TASK VERIFICATION BODY PROPOSER EXTERNAL

EXPERT

Specific Verification Protocol

Review & Approve Review

Test Plan & Audit Review & Approve Submit

Test Report Review Submit

Verification Report Review Review

Statement of Verification

Accept Review

Table 9: Breakdown of responsibility between the organisations involved with the verification

Mixergy/004/VR/NPL

18

6 REFERENCES

[1] Bristan, “Datasheet on: Deck mounted bath filler with thermostatic shower control fitting

instructions & contents list,” Tamworth, 2015.

[2] J. R. Taylor, “Independent verus dependent measurements,” in An Introduction to Error Analysis,

2nd ed., Colorado, University Science Books, 1997, p. 45–79.

[3] P. M. Armstrong, M. Uapipatanakul, I. Thompson, D. Ager and M. McCulloch, “Thermal and

sanitary performance of domestic hot water cylinders: Conflicting requirements,” Applied

Energy, vol. 131, p. 171–179, 2014.