ISO 9459-4:2013

INTERNATIONAL ISO
STANDARD 9459-4
First edition
2013-02-15
Solar heating — Domestic water heating
systems —
Part 4:
System performance characterization by
means of component tests and computer
simulation
Chauffage solaire — Systèmes de chauffage de l'eau sanitaire —
Partie 4: Caractérisation de la performance des systèmes au moyen
d'essais effectués sur les composants et par simulation sur ordinateur

Reference number
©
ISO 2013
©  ISO 2013
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Published in Switzerland
ii © ISO 2013 – All rights reserved

Contents Page
Foreword . iv
0  Introduction . v
1  Scope . 1
2  Normative references . 1
3  Terms and definitions . 1
4  Symbols, units and nomenclature . 3
5  Application . 6
6  Test method . 7
6.1  Introduction . 7
6.2  Component testing . 7
6.3  Water heater configuration for modelling . 9
7  Performance evaluation . 10
7.1  Annual task performance . 10
7.2  Weather data . 10
7.3  Thermal energy loads . 11
7.4  Thermostat set temperature . 11
7.5  Cold water inlet temperature . 11
7.6  Pump circulation control . 11
7.7  Simulation deck setup for modelling thermal stratification in storage tanks . 12
7.8  Piping configuration for solar water heaters . 13
7.9  Controllers . 14
7.10  Energy consumed for freeze protection . 14
7.11  Over-temperature control . 15
7.12  Modelling gas storage water heaters . 15
7.13  Modelling instantaneous gas water heaters . 16
7.14  Presentation of results . 17
7.15  Extension of simulation model for new products . 19
Annex A (normative) Water heater parameters . 20
Annex B (normative) Storage vessel performance . 34
Annex C (normative) Integral collector storage unit test method . 37
Annex D (normative) Corrections for effect of hail guards on solar collector efficiency . 41
Annex E (normative) PV powered DC pumps in solar water heating systems . 43
Annex F (normative) Heat exchanger test methods . 50
Annex G (normative) Reference conditions for testing and performance prediction . 58
Annex H (informative) Example simulation models . 63
Annex I (informative) Guidelines for equipment suppliers . 67
Annex J (informative) Guidelines for Certification Bodies . 68
Bibliography . 70

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 9459-4 was prepared by Technical Committee ISO/TC 180, Solar energy, Subcommittee SC 4,
Systems — Thermal performance, reliability and durability.
ISO 9459 consists of the following parts, under the general title Solar heating — Domestic water heating
systems:
 Part 1: Performance rating procedure using indoor test methods
 Part 2: Outdoor test methods for system performance characterization and yearly performance prediction
of solar-only systems
 Part 4: System performance characterization by means of component tests and computer simulation
 Part 5: System performance characterization by means of whole-system tests and computer simulation
iv © ISO 2013 – All rights reserved

0 Introduction
ISO 9459 has been developed to help facilitate the international comparison of solar domestic water heating
systems. Because a generalized performance model which is applicable to all systems has not yet been
developed, it has not been possible to obtain an international consensus for one test method and one
standard set of test conditions. It has therefore been decided to promulgate the currently available simple test
methods while work continues to finalize the more broadly applicable procedures. The advantage of this
approach is that each part can proceed on its own.
0.1 General
ISO 9459 is divided into four parts within three broad categories, as described below.
0.2 Rating test
ISO 9459-1, Solar heating — Domestic water heating systems — Part 1: Performance rating procedure using
indoor test methods, involves testing for periods of one day for a standardized set of reference conditions. The
results, therefore, allow systems to be compared under identical solar, ambient and load conditions.
0.3 Black box correlation procedures
ISO 9459-2, Solar heating — Domestic water heating systems — Part 2: Outdoor test methods for system
performance characterization and yearly performance prediction of solar-only systems, is applicable to solar-
only systems and solar-preheat systems. The performance test for solar-only systems is a “black box”
procedure which produces a family of “input-output” characteristics for a system. The test results may be used
directly with daily mean values of local solar irradiation, ambient air temperature and cold water temperature
data to predict annual system performance.
0.4 Testing and computer simulation
ISO 9459-4, Solar heating — Domestic water heating systems — Part 4: System performance
characterization by means of component tests and computer simulation, a procedure for characterizing annual
system performance, uses measured component characteristics in a computer simulation program.
Procedures for characterizing the performance of system components other than collectors are also presented
in this part of ISO 9459. Procedures for characterizing the performance of collectors are given in other
International Standards.
ISO 9459-5, Solar heating — Domestic water heating systems — Part 5: System performance
characterization by means of whole-system tests and computer simulation, presents a procedure for dynamic
testing of complete systems to determine system parameters for use in the “Dynamic System Testing
Program”. This software has been validated on a range of systems; however, it is a proprietary product and
cannot be modified by the user. Implementation of the software requires training from a test facility
experienced with the application of the product. This model may be used with hourly values of local solar
irradiation, ambient air temperature and cold water temperature data to predict annual system performance.
The procedures defined in ISO 9459-2, ISO 9459-4 and ISO 9459-5 for predicting yearly performance allow
the output of a system to be determined for a range of climatic conditions.
The results of tests performed in accordance with ISO 9459-1 provide a rating for a standard day.
The results of tests performed in accordance with ISO 9459-2 permit performance predictions for a range of
system loads and operating conditions, but only for an evening draw-off.
0.5 Introduction to ISO 9459-4
ISO 9549-4 presents a procedure predicting the annual performance of a solar thermal system using a
numerical simulation programme. The parameters of the characterisation of the thermal behaviour of the key
components such as solar collector, store and controller are derived from physical tests of the components.
Because testing of the complete system as a whole is especially expensive and time consuming for system
families, this approach offers the opportunity to determine the annual performance of a family of systems with
limited effort.
NOTE A system family is characterised by a series of hot water systems that are identical with regard to their
construction and only differ in their collector and storage dimension. An identical construction is given if the set-up of the
system is similar (pipes, electrical pump, hydraulic connections, type but not mandatorily size of the heat exchanger), the
insulation concept is similar (material, thickness) and the collectors installed are from the same type.
Procedures exist for testing most solar thermal system components. Where they exist, they are referenced. In
case no standardised component test procedures are available appropriate procedures have to be used to
determine the thermal characteristics of the components.
The intention of this International Standard is to determine the thermal performance of the system. Therefore,
it is assumed that all key components (e.g., collectors, stores, heat exchangers, etc.) used in the system are
subjected to relevant durability tests (e.g., collector qualification tests, pressurization of the collector side of
the heat exchanger, etc.) before they are tested for thermal performance.
In order to ensure a proper operation of the entire system additional durability tests may be required of the
complete system to determine operation under extreme conditions such as freezing or overheating based on
corresponding standards.
The performance evaluation procedure defined in this International Standard has been designed to provide a
means of evaluating the annual task performance of heated water systems.
This International Standard sets out a method of evaluating the annual energy performance of heated water
systems using a combination of test results for component performance and a mathematical model to
determine an annual load cycle task performance. This International Standard defines a procedure for
evaluating the task performance of conventional electric and gas domestic water heaters so that the energy
savings of solar and heat pump water heaters can be evaluated relative to conventional water heaters
operated under the same annual task load.
The performance evaluations are based on modelling annual performance in a range of climatic conditions
using a simulation program. The chosen simulation program shall have flexibility and the capacity to model the
wide range of renewable energy water heaters used worldwide.
The procedure for using this International Standard is illustrated in the figure below. The general concept is to
develop computer models that describe the performance of every component of the solar water heating
system. These models can be based on specific tests (listed herein and in EN 12977-2 and AS/NZS 4234).
These models are then combined in a system simulation that can be used to estimate the performance of the
complete solar heating system under specified hot water usage and weather conditions. Information for users
of this International Standard is presented in Annexes I and J.
vi © ISO 2013 – All rights reserved

Component Tests
- Solar collecting unit
o Active (forced circulation)
o Passive (thermosiphon, integral collector
storage)
- Storage vessels
o Solar
o Backup water heater
- Controller
- Pump(s)
Weather Solar Water Heating System Computer Hot Water Usage
Simulation Model
Estimate of:
- Energy output
- Backup fuel consumption
- Auxiliary energy consumption
- Savings compared to conventional water heating system

It is the intent of this part of ISO 9459 to be compatible with EN 12977-2, “Thermal solar systems and
components, Custom built systems, Test methods” such that tests conducted for use by certification bodies
can be done in accordance with either one interchangeably.
The terms “normative” and “informative” have been used in this International Standard to define the
application of the annex to which they apply. A “normative” annex is an integral part of an International
Standard, whereas an “informative” annex is only for information and guidance.
INTERNATIONAL STANDARD ISO 9459-4:2013(E)

Solar heating — Domestic water heating systems —
Part 4:
System performance characterization by means of component
tests and computer simulation
1 Scope
This International Standard specifies a method of evaluating the annual energy performance of solar water
heaters using a combination of test results for component performance and a mathematical model to
determine an annual load cycle task performance under specified weather and load conditions. The procedure
is applicable to solar water heaters with integral backup or preheating into a conventional storage or
instantaneous water heater and to integral collector storage water heaters.
System operating requirements specified in this International Standard are for the purpose of determining an
annual performance rating for domestic water heaters. There are no product design or operation requirements
in this International Standard.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 9488:1999, Solar energy — Vocabulary
ISO 9806 (all parts), Test methods for solar collectors
EN 12977 (all parts):2012, Thermal solar systems and components — Custom built systems
EN 12975 (all parts):2006, Thermal solar systems and components — Solar collectors
EN 12976 (all parts):2006, Thermal solar systems and components — Factory made systems
AS 1056.1:1991, Storage water heaters — General requirements
AS/NZS 2712:2007, Solar and heat pump water heaters — Design and construction
AS/NZS 2535:2007, Test methods for solar collectors
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 9488 and the following apply.
There are two differences with ISO 9488-1999:
 auxiliary energy is used here to represent energy consumed by pumps, fans, and controls in a solar
heating system
 backup energy refers to energy contributed by a source other than solar
3.1
tilt angle
angle between the absorbing surface of the collector and the horizontal
3.2
container
vessel including fittings, in which the heated water is stored; sometimes referred to as a store, storage
container, cylinder, storage vessel, or tank
3.3
electricity supply options
3.3.1
continuous
continuously available electricity supply
3.3.2
limited time of supply
electric supply available at limited times, as follows:
3.3.2.1
night rate
electricity supply at restricted night hours (see Annexes A and B for typical availability times)
3.3.2.2
extended off-peak
electricity supply during extended hours (see Annexes A and B for typical availability times)
3.4
dual element tanks
tanks incorporating dual electric elements at different levels in the tank
Note: Each element may be connected to a different electric supply or be operated under local control.
3.5
heat pump water heater, solar assisted
a system incorporating a compressor (vapour compression), an evaporator exposed to ambient air or solar
radiation, or both, a condenser and a water container heated either directly or indirectly by the condenser
3.6
one-shot backup
operation of a backup heat source for one heating cycle
Note: The heater is returned to normal control after one heating cycle.
3.7
factory-made solar water heater
package systems are batch products, sold as complete and ready to install kits with fixed configurations
Note: Systems of this category are considered as a single product and assessed as a whole.
2 © ISO 2013 – All rights reserved

3.8
reference water heater
a conventional water heater used to define annual purchased energy use for the purpose of computing energy
savings of other products
3.9
tank nodes
horizontal sections of a storage tank
Note: Used in the simulation program to model thermal stratification in the storage tank.
3.10
tank modelling options
3.10.1
fixed inlet positions
fluid flows into the storage tank are considered to mix with the contents of the tank node at the same level with
the inlet fitting
3.10.2
variable inlet positions
fluid flows into the storage tank are considered to rise or fall to the level of the tank node with the closest
temperature to the inlet flow
3.11
computer simulation model
TRNSYS or equivalent computer program used to simulate the performance of solar water heating
components and systems
4 Symbols, units and nomenclature
Symbol Units Meaning
a, b, c [-] correlation coefficients for solar collector efficiency
B [MJ/year] annual energy used by a conventional water heater
c
B [MJ/year] annual electrical energy used by water heater
e
B [MJ/year] annual gas energy used by water heater
g
B [MJ/year] annual backup energy for a solar or heat pump water heater
s
C [kJ/(kg C)] heat capacity
p
C [-] modified capacity ratio
rmod
DE [MJ] difference in energy content of a tank between the start and the end of
a heat loss or warm-up test
d [m] diameter
e [W] electrical power use during burner operation
n
e [W] electrical power use during standby
sb
E [%] thermal efficiency of gas water heater
E [MJ/year] integrated energy added to tank from gas combustion
g
f [-] purchased energy savings relative to a conventional water heater
R
(BcBs)/Bc
2]

G [Wm beam irradiance on the collector aperture
b
2] 4
G [Wm net long wave irradiance (λ>3µm) (Gs  σT )
L a
2]
G [Wm extraterrestrial solar irradiance
o
G [-] Grashoff number
r
2]
G [Wm long wave thermal irradiance (λ>3µm)
S
2]
G [Wm hemispherical solar irradiance on the collector aperture
T
h [W/m K] heat loss coefficient
K [-] solar collector angle modifier
τα
K , K [-] bi-axial incidence angle modifiers
NS EW
K [-] average clearness index for specified test
T
M [MJ/h] maintenance rate [standing or stand-by heat loss rate from a heated
water tank]
M [kg] mass withdrawn from the storage vessel
drawn
M [kg/s] mass flow rate
rate
M [kg/s] mass flow rate through a system
sys
M *C [kJ/K] capacitance
tank p
n [-] Julian day of the year (1-365)N [-] number of events
P [Pa] pressure drop
Pr [-] Prandtl number
Q [kJ] energy purged from the system after the wait or irradiance period
del
Q [Lj] initial charge energy of a tank or system when subjected to an
initial
instantaneous purge between two set temperatures.
R [MJ/h] determined gas consumption
Re [-] Reynolds number
t [h:min:s] duration of test
Time [s] decay test duration
decay
T [°C] temperature
T [°C] mean collector fluid temperature
4 © ISO 2013 – All rights reserved

T [°C] ambient temperature
a
T [°C] annual average ambient outdoor air temperatureT [K] thermostat dead
a, avg db
band
T [°C] temperature of the water delivered from the DHW system at the outlet
del
of the system
T [°C] collector inlet fluid temperature
i
T [°C] mean temperature of water in the storage tank
W
T [°C] environment temperature (ie. temperature of air surrounding the
env
storage vessel)
T [°C] average environment temperature
env ave
T [°C] temperature of the water in a test system at the beginning of a
high
high-temperature test (typically 55  60 °C)
T [°C] temperature of the liquid entering the storage vessel
in
T [°C] temperature of the water in the test system at the beginning of a test
initial
(heat loss or warm-up)
T [°C] temperature of the water in a test system at the beginning of a
low
low-temperature test (preferably near ambient temperature)
T [°C] average tank fluid temperature
tank ave
T [°C] final average tank fluid temperature after the decay or irradiation, period
tank ave final
T [°C] initial average tank fluid temperature before the decay or irradiation
tank ave ini
period
T [°C] final average tank fluid temperature after the purge period. This value is
tank ave purge
usually estimated by averaging the tank inlet temperature and T
del.
∆t [s] change in time
T [°C] manufacturer's recommended thermostat setting
set
T [°C] sky temperature
sky
-1
u [ms ] surrounding air speed over the solar collector (or evaporator)
U [W/m K] heat loss coefficient
UA [W/K] heat loss coefficient  area product for the tank
UA total UA loss of the storage tank from a decay test with the system
isolated loss total
piping installed
UA installed total UA loss of the storage tank from a decay test
installed loss total
α [-] unglazed collector hemispherical absorber absorptance, short wave
 [-] slope or tilt, the angle between the plane of the collector and the
horizontal
ε [-] unglazed collector hemispherical absorber emittance, long wave
 [-] modified effectiveness
mod
η [-] solar collector efficiency
 [-] latitude, the angular location north or south of the equator
θ [rad] incidence angle relative to the collector aperture normal
 [-] orientation angle, the direction which a collector faces, expressed as
the azimuth angle of the horizontal projection of the surface normal
 [W/mK] thermal conductivity
5 Application
The procedure in this International Standard uses a mathematical model to assess annual energy task
performance; hence the application of the procedure is restricted only by the availability of suitable
mathematical models. Weather data and typical modelling data files are specified in Annex G. The operating
conditions and product configurations to be used for evaluating the energy performance of a water heater are
specific to certification or incentive programs and are not defined in this International Standard.
This International Standard can be applied to solar water heaters with the following:
a) flat plate, concentrating or evacuated tubular solar collectors,
b) thermosiphon or forced fluid circulation through the solar collectors,
c) collector loop heat exchangers,
d) systems for combined domestic hot water preparation and space heating (combisystems),
e) integral collector storage,
f) horizontal or vertical water storage tanks,
g) storage with one or more electrical heating elements,
h) storage tanks with internal gas backup heaters,
i) solar preheat systems in series with instantaneous water heaters,
j) solar thermal systems combined with heat pumps (e.g., solar collectors acting as the refrigerant
evaporator).
Other water heater configurations incorporating the above components may also be modelled.
For limited time-of-supply electric storage water heaters, the temperature stratification in the storage tank is
evaluated throughout the day and used to quantify the variation of tank heat loss with time, due to cooling of
the bottom of the tank. Mixing during load draw off and conduction between the hot and cold layers in the tank
is also included. All storage tanks shall be rated for standing heat loss and maintenance rate. The operational
heat loss accounting for non-uniform insulation around the tank and thermal stratification in the tank is
determined by the annual load cycle performance model.
6 © ISO 2013 – All rights reserved

6 Test method
6.1 Introduction
This International Standard defines a means of evaluating the purchased energy use of water heaters
operating under specified weather and load conditions.
For solar water heaters, this standard can be used for any system type that can be reasonably modelled in a
computer simulation model. The performance of individual components is evaluated under ISO 9806, AS 4552,
AS/NZS 4692.1, AS 1056.1, EN 12975, EN 12976, and EN 12977. The performance of heat pump water
heaters with evaporators exposed directly to solar radiation is evaluated using test results for the evaporator
evaluated under ISO 9806 and performance of the compressor is evaluated under ASHRAE Standard 23-93.
The purchased energy use calculated is only representative of the product model described in Annex A.
6.2 Component testing
6.2.1 Storage vessels
There are a wide variety of storage vessels available. They range from simple single-wall tanks to double-wall
(mantle) vessels with multiple integral heat exchangers and backup energy input. As the complexity of the
vessel increases, so does the complexity of the method necessary for adequately characterizing the vessel.
6.2.1.1 Simple storage tanks
The standing heat loss of simple storage vessels without a heat exchanger and/or backup energy input may
be evaluated in accordance with Annex B or in accordance to 6.3.1.4.1 of EN 12977-3.
6.2.1.2 Complex storage vessels
6.2.1.2.1 Storage vessels with electric backup heating
The standing heat loss of complex storage vessels may be evaluated using EN 12977-3.
NOTE Electric backup heaters in solar water heater storage tanks may be located above the bottom of the tank to
separate the functions of solar and backup heating. Evaluation of the standing loss and rated delivery of such tanks
requires a test with a special electric element fitted in the bottom of the tank to minimize thermal stratification in the tank
during the standing heat loss test.
6.2.1.2.2 Storage vessels with gas backup
The following performance factors may be evaluated using the test methods in AS 4552.
a) thermal efficiency (%),
b) determined gas consumption (MJ/h),
c) maintenance rate (MJ/h),
d) electric power usage during standby (W),
e) electric power usage during burner operation (W).
6.2.1.2.3 Storage vessels with raised level gas backup heating
Heat loss from storage tanks incorporating raised level gas backup heating from either an internal or external
burner shall be determined by direct measurement using a specially configured tank that is maintained at
uniform temperature during heat loss testing.
6.2.1.2.4 Instantaneous gas water heaters
The following performance factors may be evaluated using the test methods in AS 4552.
a) thermal efficiency (%),
b) output (kW),
c) pilot gas consumption (MJ/h),
d) start-up heat capacity (MJ/event),
e) electric power usage during standby (W),
f) electric power usage during burner operation (W).
6.2.1.2.5 Solar assisted heat pump storage water heaters
The heat pump compressor thermal capacity and power consumption shall be evaluated for evaporator
refrigerant temperatures from 5 °C to 30 °C (at least four temperatures) and condenser refrigerant
temperatures from 30 °C to 70 °C (at least four temperatures) using the test procedure in ASHRAE 23-93.
The standing heat loss of the storage vessel in a heat pump water heater shall be evaluated using
AS/NZS 4692.1, EN 12977-3, or the procedure described in Annex B.
6.2.2 Flow rate in pumped collector-loops
The flow rate in forced circulation systems shall be determined using one of the following. The length of pipe
shall be the longest length allowed by the manufacturer's published installation instructions for the system. In
absence of such specifications, the total pipe length should be set according to Annex G, Reference
conditions.
6.2.2.1 Constant speed pumps
6.2.2.1.1 Measurement
A system shall be assembled with a specified length of pipe of the manufacturer's specified diameter each
way between the tank and the collector array. For systems with site specific flow adjustment, the flow control
device specified by the manufacturer for this piping length shall be installed. Flow rate shall be measured
under normal operating conditions.
6.2.2.1.2 Calculation
The flow rate may be calculated using analytical functions for the pressure drop of each fitting normally used
in the system, a specified length of pipe of the manufacturer's specified diameter each way between the tank
and the collector array, and the published head-flow characteristics of the pump.
8 © ISO 2013 – All rights reserved

6.2.2.2 Variable speed pumps
6.2.2.2.1 Measurement
A system shall be assembled with a specified length of pipe of the manufacturer's specified diameter each
way between the tank and the collector array. For systems with site specific flow adjustment, the flow control
device specified by the manufacturer for this piping length shall be installed. Flow rate shall be measured
under normal operating conditions and characterized as a function of a control variable. This control variable
could be a temperature difference, solar radiation, or any other parameter specified in the system design.
6.2.2.2.2 Calculation
The flow rate may be calculated using analytical functions for the pressure drop of each fitting normally used
in the system, a specified length of pipe of the manufacturer's specified diameter each way between the tank
and the collector array, and the characteristics of the pump speed controller. Flow rate shall be calculated
under the rating conditions and characterized as a function of a control variable. This control variable could be
a temperature difference, solar radiation, or any other parameter specified in the system design. Refer to
Annex E for a method to evaluate DC pumps powered by a photovoltaic module.
6.2.3 Solar collectors
6.2.3.1 Solar collector efficiency
The efficiency of solar collectors shall be evaluated using ISO 9806.
6.2.3.2 Collector efficiency correction for shadowing due to impact guard
If an impact guard is added to a collector after the collector efficiency test then the collector efficiency shall be
corrected for
a) normal incidence shadowing due to the hail guard,
b) incidence angle modifier of hail guard as specified in Annex D.
6.2.3.3 Integral collector storage units and other designs not addressed above
The efficiency of integral storage units (including non-separable thermosiphon) and other designs not
addressed above shall be evaluated using EN 12976 or Annex C.
6.2.4 Heat Exchangers
All heat exchangers shall be tested in accordance with one of the methods given in Annex F or the system
parameter identification method included in the storage vessel test described in EN 12977-3, chapter 6.3.1.5.
6.2.5 Controllers
All controllers shall be tested in accordance with the method given in EN 12977-5.
6.3 Water heater configuration for modelling
The system configuration data listed in Annex A shall be used in the modelling; additional data may be
needed for some systems.
The solar collector tilt angle and azimuth shall be set according to Annex G, Reference conditions.
One of the following backup supply and control options shall be specified in the performance model:
a) Single or dual electric elements with each element operated on different electric supply options.
b) Gas in-tank backup heating.
c) Gas or electric storage water heater in series with solar or heat pump preheater.
d) Series instantaneous backup with the solar heater operating as a preheater.
e) In-tank heat exchanger connected to an external water heater.
The timing of electrical backup heating may be set to night rate tariff times, extended off-peak times,
continuous backup or set by a local controller. The backup heater controller may vary backup input to the tank
in response to solar radiation conditions, stored energy level in the tank, load demand and other conditions.
If the electric supply utility allows user activation of one-shot backup heating in time limited electric tanks then
the effect of this may be modelled as part of the annual performance evaluation.
User over-ride of backup heating shall not be included in the modelling.
7 Performance evaluation
7.1 Annual task performance
The mathematical model to be used for the annual task performance evaluation shall be a validated simulation
program suitable for the product type being modelled. See Annex G for validation requirements.
The annual load cycle performance of water heaters shall be determined with a computational time step of
0.1 hours or less to determine the annual backup energy needed for the specified load and environmental
conditions.
For components whose modelling parameters were derived from computer modelling, the same computer
model that was used to derive the modelling parameters from the original component test data shall be used
when evaluating annual task performance. Note that modelling parameters can also be defined directly from
test data without computer modelling.
7.2 Weather data
The weather data used for the simulation shall include records for the locations specified in 7.14. The
performance shall be based on hourly values for the following variables:
Table 1 — Hourly value variables
Variable Applicability
Ambient temperature All systems
Global irradiation Solar water heaters and solar-assisted heat pumps
Beam irradiation Solar water heaters and solar-assisted heat pumps
Wet bulb temperature Heat pumps only
Wind speed Solar-assisted heat pumps and solar water heaters incorporating
unglazed collectors
Cloud cover Solar-assisted heat pumps and solar water heaters incorporating
unglazed collectors
Infrared radiation or sky temperature (if available) Solar-assisted heat pumps and solar water heaters incorporating
unglazed collectors
10 © ISO 2013 – All rights reserved

7.3 Thermal energy loads
The peak daily thermal energy loads and seasonal and daily variations of load specified in Annex G shall be
used for the annual task performance evaluation. The load is distributed within each day using a daily load
pattern and the load is varied each month using a seasonal load pattern. The simulation input deck shall be
configured so that the load is specified in terms of energy withdrawn after the tempering valve in the manner
shown in the example simulation deck files.
7.4 Thermostat set temperature
7.4.1 Storage water heaters
For the purpose of determining annual performance rating, the model shall simulate heating of water above
the lowest thermostat to the temperature specified in Annex G.
7.4.2 Instantaneous supplementary heaters
The set temperature of instantaneous backup heaters in series with solar or heat pump storage preheaters
shall be as specified in Annex G.
7.4.3 Minimum delivery temperature
The system shall be capable of satisfying a minimum delivery temperature of 45 °C under peak winter load
conditions for the specified no-solar operating conditions.
To quantify the backup heater capacity, the delivery temperature under no-solar conditions shall be checked
by monitoring (recording) the delivery temperature calculated during each load draw-off for peak-winter load
conditions. The no-solar simulation shall be continued over a period of ten or more days until stable operation
is obtained under the no-solar conditions specified in Annex G. A minimum delivery temperature of 45 °C shall
be achieved for every delivery.
If a system fails to meet the minimum delivery temperature of 45 °C then it may be rated for a lower load
provided it can satisfy the minimum delivery temperature requirement for the lower load. The load used for
rating shall be stated in the report.
7.5 Cold water inlet temperature
The cold-water inlet temperature profile specified in Annex G shall be used.
7.6 Pump circulation control
If the stored potable water is pumped through the solar collector or an external heat exchanger then the
circulation flow rate may have a significant effect on thermal stratification in the storage tank. The effect of a
high flow-rate may be particularly significant in single tank systems where a backup heater heats the top
section of the tank. If the pumped loop return level to the tank is below the level of the in-tank backup heater
then disturbance of thermal stratification due to forced circulation through the storage tank can be minimized
by using a low flow rate.
7.6.1 Low-flow criteria
Under low-flow operation the collector flow into the tank may be considered to promote thermal stratification in
the tank. To satisfy the low-flow criteria, the pump control or pressure drop in the collector-loop shall be
adjusted to suit the collector array size and pipe lengths and fittings of each installation. Low-flow shall not be
included in the system model unless the system design includes specific flow control(s).
The criteria for modelling the potential for thermal stratification in a forced circulation solar water heater are
a) flow rate less than 0.75 l/(min m aperture),
b) collector flow return level to the tank is below the level of the backup heater.
7.6.2 Controlled-flow pumped-circulation
Forced circulation solar water heaters that use collector loop flow rate control to set a flow rate less than
0,75 l/(min m aperture) for each installation shall use the controlled-flow thermal stratification requirements in
7.7.1 for load cycle performance rating.
7.6.3 Uncontrolled-flow forced circulation
Forced circulation solar water heaters that do not have site specific adjustment or control of collector loop flow
rate shall use the uncontrolled-flow thermal stratification requirements in Clause 7.7.3 for load cycle
performance rating.
7.6.4 External collector loop heat exchanger systems
Solar water heaters using a pumped collector loop working through an external heat exchanger (sidearm heat
exchanger) with a thermosiphon loop on the tank side of the heat exchanger shall be modelled as for
thermosiphon systems or with natural convection loop routines.
7.6.5 Pump controllers
The simulation deck shall model the operation of pump controllers that sense flow rate, collector temperature
rise or other real-time variables in order to set the flow rate in the collector-loop or sidearm-loop.
7.7 Simulation deck setup for modelling thermal stratification in storage tanks
Thermal stratification in a solar heated storage tank depends on the flow rate between the tank and the solar
collector or collector loop heat exchanger. Thermal stratification in the storage tank shall be modelled as
mixed flow or stratified flow using the following system classifications:
7.7.1.1 Controlled flow-rate forced circulation
Thermal stratification in the storage tank of products that satisfy the low-flow collector-loop criteria and have
collector-loop flow rate adjustment for each installation shall be modelled using the following options in the
simulation model:
a) site specific flow rate specified by the manufacturer or set by the controller,
b) variable inlet position mixing option for the tank,
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c) 20 or more tank nodes in a fixed node tank model or an automatic node model or the Kleinbach method
of determining the number of nodes.
7.7.2 Thermosiphon circulation
Thermal stratification in the storage tank of thermosi
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