02- Manual ADEME, Aquecimento Solar

Prévia do material em texto

Agence de l’Environnement
et de la Maîtrise de l’Energie
A D E M E
 
 
 
Solar Hot Water 
 
A manual for designing, sizing and installing 
collective systems 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
April 2002 
 
 
 
 
 
 
 2
Summary 
 
This manual for designing, sizing and installing solar systems for domestic hot water 
production specifies the basic rules, that have been tested and drawn from the 
experience gained by monitoring system performance and following five essential 
principals: 
§ Simple installation, 
§ Safety in use, 
§ Integration in buildings, 
§ Efficient performance, 
§ Monitoring. 
 
Particular importance is given to global system design, solar collector integration and 
sanitary regulation changes. 
 
The choice of the components and their installation following professional codes of 
practice is completed by recommendations and detailed comments concerning the sizing 
of the different parts of the installation as well as the calculation of the provisional 
performance. 
 
 3
Contents 
Solar Hot Water .................................................................................................1 
Summary ..........................................................................................................2 
Contents...........................................................................................................3 
1. Introduction...................................................................................................4 
2. General principals ...........................................................................................5 
2.1 Basic climatic data ......................................................................................5 
2.1.1 Solar radiation.....................................................................................5 
2.1.2 Temperature of the water supply ............................................................8 
2.1.3 Wind and snow .................................................................................. 10 
2.2 The needs for domestic hot water.................................................................. 14 
2.2.1 Domestic hot water supply temperature ................................................. 14 
2.2.2 Analysis of the demand ....................................................................... 13 
2.3 Heating water with solar energy .................................Erreur ! Signet non défini. 
2.3.1 Collection ......................................................................................... 18 
2.3.2 Energy transfer and storage................................................................. 23 
2.3.3 Auxiliary heating................................................................................ 31 
3. The project procedure ................................................................................... 32 
3.1 Preliminary studies...................................................................................... 32 
3.1.1 Estimating the needs for hot water ....................................................... 32 
3.1.2 Site insolation ................................................................................... 33 
3.1.3 Collector layout study ......................................................................... 34 
3.1.4 Collector connections.......................................................................... 44 
3.1.5 Piping .............................................................................................. 46 
3.1.6 Storage and backup supply .................................................................. 48 
3.2 Detailed studies .......................................................................................... 52 
3.2.1 Sizing the solar equipment .................................................................. 52 
3.2.2 Estimating solar system performance .................................................... 55 
3.2.3 Tools for solar DHW system performance calculations............................... 60 
3.2.4 Methodology for system definition and sizing .......................................... 63 
3.2.5 Preliminary system sizing .................................................................... 68 
3.2.6 Ajustment of the data concerning collectors and storage........................... 83 
3.2.7 Sizing the heat exchanger ................................................................... 84 
3.2.8 Sizing the primary circuit : pipes and pumps........................................... 86 
3.2.9 Sizing the safety equipment................................................................. 88 
3.3 Project estimation ....................................................................................... 90 
3.3.1 Technico-economic estimation.............................................................. 90 
3.3.2 Guaranteed Solar Results.................................................................... 92 
3.3.3 Environmental impact......................................................................... 94 
4. System management and maintenance ............................................................ 95 
4.1 Filling up ................................................................................................... 95 
4.2 Starting up ................................................................................................ 95 
4.3 Commissioning ........................................................................................... 95 
4.4 Periodic maintenance .................................................................................. 99 
4.4.1 Periodicity and maintenance actions ...................................................... 99 
4.4.2 Justification of the controls and maintenance actions...............................100 
4.4.3 Limits to the maintenance service........................................................101 
4.5 Tele-monitoring .........................................................................................102 
5. Further information......................................................................................104 
6. Examples of collective systems ......................................................................105 
 
 4
1. Introduction 
 
Using the sun to provide domestic hot water seems to be a perfectly logical solution. The 
principle of a solar water heater is simple and the technology is both well known and 
reliable. 
 
At a human level, solar energy is pollution free, inexhaustible, env ironment friendly and 
safe. It helps to save energy resources, without producing waste or emitting polluting 
gas, such as carbon dioxide. 
 
Over and above the issues concerning the environment and the impact on the 
atmospheric greenhouse effect, hot water supply represents a considerable part of the 
energy bill in buildings, which can be reduced by using solar energy. 
 
Thermal solar technology has been constantly improved over the last 20 years and has 
now reached a high level of maturity. More than 500.000 m2 of solar collectors have been 
installed in France (on the main land and overseas) and all the applications for which 
they are used have good references. 
 
High quality products are available, the thermal systems are reliable and their 
performance can be guaranteed, due to: 
§ Qualification procedures and equipment certification (Technical reports, CSTBat 
labelling), 
§ Calculation and sizing tools (SOLO, Polysun, TRNSys, PSD-Private house software…), 
§ System control and monitoring (tele-monitoring). 
 
The conditions needed for good and durable system operation have been established 
progressively and within the framework of the "GRS" contract (Guaranteed Solar Results) 
the guarantees offered for collective applications aresignificant. 
 
Including the heating of swimming-pools, which only concerns 15% of the collector 
surface area installed at present, the amount of solar energy used for supplying hot 
water and heating buildings using active systems in France (without the territories 
overseas) amounts to 750 Terajoules (or 208 GWh). 
 
 
 
 5
2. General principals 
2.1 Basic climatic data 
2.1.1 Solar radiation 
 
The sun is a free and clean source of energy, the average radiation in France is estimated 
to be greater than 1 000 kWh/m2/year, which represents a potential annual energy 
supply for the whole country of more than 50.000 million tons of oil equivalent (toe). 
 
 
Average daily global radiation in kWh/m2 throughout the year. 
(facing South, slant equal to the latitude) 
Source : European Atlas of solar radiation. Volume II: Inclined surfaces. W. Palz, Commission of 
the European Communities; Directorate General Science, Research and Development. 
 
 
Solar radiation is a thermal radiation that is diffused in the form of electromagnetic 
waves. Outside the earth's atmosphere it supplies a virtually constant source of energy 
equal to 1.370 W/m2, this is called: solar constant Ics. 
 
In order to reach the surface of the earth, the solar radiation passes through the 
atmosphere where part of the solar energy is dispersed by: 
- Molecular diffusion (particularly the U.V. radiation) 
- Diffuse reflection on the atmospheric aerosols (water drops, dust…) 
- Selective absorption by the atmospheric gasses. 
 
The corresponding reduction of the solar radiation depends on the thickness of the 
atmosphere that must be crossed and this depends on the latitude of the site and the 
time. 
 
 
 
 6
 
 
Propagation of solar radiation through the atmosphere 
 
 
 
Solar radiation is direct, before getting to the earth's atmosphere. It appears to be a 
beam of virtually parallel light. Only part of this direct radiation crosses through the 
atmosphere and reaches the ground. Another part of the radiation is diffused and takes 
off in all directions. 
 
When this diffuse radiation reaches the ground, it appears to come from all directions of 
the sky. 
 
Global radiation on the earth's surface is the sum of the: 
§ Direct radiation, that has crossed through the atmosphere, 
§ Diffuse radiation, coming from all directions of the sky. 
 
Therefore, an exposed surface receives both direct and diffuse radiation, and also part of 
the global radiation reflected by nearby objects, particularly the ground, for which the 
reflection coefficient is called "albedo". 
 
 7
 
Solar radiation at the earth's surface 
 
 
 
 
 8
2.1.2 Temperature of the water supply 
 
In France, 70% of the drinking water comes from confined groundwater, by means of 
wells or springs. The rest is surface water: lakes and rivers. All these sources are 
renewable: the rain and the snow keep them filled with 200 billion m3 per year. 
 
The energy needed to provide domestic hot water depends twofold on the temperature of 
the cold water supply: the colder the water, the more energy will be needed to heat it to 
a given temperature (storage requirements, for example), and more hot water will be 
needed, in volume, in order to ensure a constant temperature when mixed with the cold 
supply. 
 
 
 9
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Average monthly cold water supply temperature (Source : Tecsol) 
 
 
 10 
2.1.3 Wind and snow 
 
2.1.3.1 Wind 
 
In meteorology, an average wind is considered to be the average over a period of 10 
minutes, 10 metres above the ground. The French meteorological journals always refer 
to average winds. However, gusts of wind can be more than 50% greater than the 
average figures. 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 4: histogram of wind speeds 
 
The direction of the wind and its strength are the two characteristics that describe a 
horizontal wind. In meteorology, one always refers to the direction from which the wind 
comes with reference to the cardinal points (North, East, South, West) or to the 
difference in angle from the North (for example, a South wind is in the 180° sector, a 
West wind is in the 270° sector). 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 5: Compass Rose 
 
Speed (m/s) 
F
r
e
q
u
e
n
c
y 
West East 
South 
North 
 
 11 
2.1.3.2 Snow 
 
Snow can affect buildings in several ways and particularly on the roof. An important 
snowfall can cause the roof to collapse. Ice barriers can lead to leaks from under the 
shingles or joints. Sliding snow on a sloped roof or skylight can be dangerous for 
pedestrians. Water can penetrate in to a building due to the infiltration of snow blown by 
the wind. 
 
The load due to snow on the roof varies with the different regional climates. It can also 
vary in relation to the aspect and the form of the roof. 
 
 No wind 
Wind 
Wind 
Protection Snow pushed by the wind 
Un balanced load Snow after sliding 
Wind or no wind 
 
 12 
2.1.3.3 Estimation of climatic loads 
 
Snow and wind are two natural factors that require appropriate building design for the 
occupants' safety and comfort. 
 
The regulations NV 65 define the effect of snow and wind on buildings. 
 
The new edition (March 2001) includes the latest modifications and makes the link 
between the indications shown in the DTU 40 and 43, concerning roof construction, 
waterproofing and the snow and wind regulations in vigour. 
 
The wind map given in the regulations is in transition towards the Eurocode that 
describes the wind speed on a basis of probability: "50 year" wind. 
 
 
 
 13 
2.2 The needs for domestic hot water 
 
2.2.1 Domestic hot water supply temperature 
 
Domestic hot water supply systems using solar energy require an auxiliary source of 
energy, for the following reasons: 
§ Maintaining the required water temperature for the hot water needed, as the solar 
installation is generally sized to cover only part of the needs. 
§ Maintaining the required water temperature in order to avoid bacteria, particularly 
legionels. 
Generally speaking, in order to limit the development of these bacteria, water 
stagnation in dead-end piping should be avoided. The temperature of the hot water 
leaving the storage tanks should be at least 60°C, and in the case of a circulation 
loop, the return temperature should be at least 50 °C. 
 
The users should always be protected from scalding at the supply points, where the 
water temperature should not be greater than 50°C. 
The directives DGS 97/131, dated 24th April 1997 and DGS 98/771, dated 31st 
December 1998, define the codes of practice for the water supply mains maintenance in 
health establishments, the disinfection of piping and the safety measures for systems 
where the risks are high and in buildings open to the public. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
2.2.2 Analysis of the demand 
 
Canne plongeante
Ballon
Témoin
Manchette
M
MA
MA
CHAUDIERE
SOLAIRE
Ballon
Canne plongeante
D'APPOINT
CIRCUIT
SOLAIRE
RECHAUFFEUR
DE BOUCLE
normalement ouverte
Vanne à boisseau sphérique 
normalement fermée
Vanne à boisseau sphérique 
MitigeurM
Clapet anti retour
Soupape de sécurité
à émetteur d'impulsions
Compteur volumétrique 
Filtre à tamis
Manomètre
MA
Circulateur
EC . 50°C
Bouclage
EF
.frtecsol
André-JOFFRE
Schéma n° 01 31-05-2001
BP 434 - Technosud 
Perpignan Cédex
Tel : 04-68-68-16-40
Fax : 04-68-68-16-41
Schéma d'appoint
 
Example of a back up heater separated from the boiler, a circulation loopis used for the 
distribution with a re-heater used for temperature maintenance 
 
 14 
Hot water production is amongst the most efficient solar energy applications, particularly 
for collective systems in residential and tertiary buildings where the hot water demand is 
important and steady; notably, for collective housing, hotels and health establishments. 
 
The present requirements for collective buildings (housing, hotels, hospitals,..) show a 
growing demand for hot water, not only for the sanitary needs but also for the domestic 
tasks. The usefulness of a hot water supply system is characterized by the availability of 
the hot water, in a sufficient quantity, at a given temperature, when it is needed and at a 
cost as low as possible. 
 
DHW requirements in collective housing 
 
Hot water supply and heating represent the highest service expense connected with 
housing. In social housing it can vary from 300 to 760 euros per annum for a floor area 
of 65 m2. 
Depending on the region, solar energy can be used to cover from 40 to 70% of the 
needs. 
There are 10 million collective dwellings in France, 3 million of these are considered to be 
for social housing. 
 
A few figures 
 
In the residential sector, the energy load for a building, can be calculated from the 
following equation: 
 
Becs = SS 1,16 . Vecs . DD T . lp 
 
Where: 
Becs = sum of the energy consumption in each dwelling (Wh) 
Vecs = 35 litres of hot water per day and per occupant. 
DT = 45 K 
lp = number of occupants in the dwellings 
 
 
 
Number of rooms in the dwelling 1 2 3 4 5 
Consumption (litres/day) at 60°C 40 55 75 95 125 
 
Monthly distribution of the consumption 
Use Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec 
 1,25 1,20 1,10 1,05 1,00 0,80 0,50 0,60 0,90 1,05 1,15 1,40 
(Source EDF: Eau chaude électrique Résidentiel et Tertiaire – March 1987) 
 
The following figures were measured in about 700 dwellings 
 Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec 
Litres/day/dwelling 
at 55 °C 
119 116 113 112 105 99 85 75 100 104 117 119 
(Source EDF: Eau chaude électrique Résidentiel et Tertiaire – March 1987) 
 
 
 
 
 
 
DHW requirements in hotels 
According to the hotel category, the daily hot water consumption varies from 70 to 160 
litres per room and from 8 to 15 litres in the kitchen per meal. 
 
 15 
The clientele is more and more attached to environmental protection and a solar system 
contributes to the good image of a hotel. However, the value of a solar system is highly 
dependant on the frequentation of the establishment. 
 
Some indicative figures: 
 
Type of establishment Equipment Water consumption at 60°C 
Hotel 1 * Collective shower 
(1 for 4 rooms) 
70 l / day /room 
Hotel 2/3 * Bath 100 -140 l / day /room 
Hotel 4/5 * Bath + shower 160 l / day /room 
Hotel 2 * Snow Bath 160 l / day /room 
(Source: Calculs pratiques de plomberie sanitaire. Editions Parisiennes) 
 
 
DHW requirements in litres/ day /room at 60°C 
Use Jan Feb Mar Apr May Jun July Aug Sept Oct Nov Dec 
4 seasons 
Summer holidays 
Winter holidays 
66 
0 
39 
61 
10 
100 
60 
12 
50 
57 
56 
100 
61 
64 
50 
82 
81 
75 
97 
92 
94 
98 
100 
94 
100 
77 
56 
100 
46 
0 
78 
0 
0 
77 
0 
12 
 
Corrective coefficient to be applied 
Number of stars Without 
0,65 
* 
0,75 
** 
1,00 
*** 
1,35 
**** 
1,50 
Place Mountain 
1,35 
Sea 
1,00 
Country 
1,00 
Town 
1,00 
 
Presence of a 
laundry 
Yes 
1,25 
No 
1,00 
 
(Source EDF : Eau chaude électrique Résidentiel et Tertiaire – March 1987) 
 
 
 
Health establishments and homes for the elderly 
The health system in France is dense and varied. 1061 public establishments work in 
association with 2.721 private hospitals and clinics, making a total of 3.782 
establishments, with 667.000 beds and reception rooms. 
In 1995 and in all sectors, elderly people were accommodated in 9.550 homes 
representing a surface area of 19 millions m2 and about 565.000 beds. 
These establishments have important hot water requirements, that are fairly steady 
throughout the year. The daily consumption is about 60 litres per bed, to which the 
kitchen needs (8 to 15 litres per meal) and the laundry (6 litres per kg of laundry) must 
be added. 
 
Some indicative figures: 
 
Type of establishment Equipment Water consumption at 60°C 
Hospital and clinic Without restaurant or 
laundry 
60 l / day / bed 
Elderly persons home Without restaurant or 
laundry 
60 l / day / bed 
(Source : Calculs pratiques de plomberie sanitaire. Editions Parisiennes) 
 
 
Other establishments 
 
Type of establishment Equipment Water consumption at 60°C 
Hostel Wash basin + shower 60 l / day/room 
 
 16 
(Single rooms) Collective WC 
Collective kitchen 
School Majority for mid-day 
meal 
5 l / day /student 
Barracks, boarding school Without restaurant and 
laundry 
30 l / day /person 
Camp site 4 * Collective bathrooms + 
dish washing 
60 l / day /place 
Factory (Dressing room) Hot water for employees 
only 
20 l / day /person 
Office 5 l / day /person 
(Source: Calculs pratiques de plomberie sanitaire. Editions Parisiennes) 
 
 
 
Other applications (to be added where required) 
 
Type of establishment Equipment Water consumption at 60°C 
Gymnasium Check on the sports 
practiced (Football or 
rugby: + 50 %) 
30 l / user 
Restaurant Ordinary meal 
High class meal 
Breakfast 
8 l / meal 
12 to 20 l / meal 
2 l / meal 
Canteen Kitchen for reheating 
Normal meal 
3 l / meal 
5 l / meal 
Laundry Hotel 4/5 * 
Short cycle 
Automatic cycle 
7 l / kg laundry 
6 l / kg laundry 
5 l / kg laundry 
(Source: Calculs pratiques de plomberie sanitaire. Editions Parisiennes) 
 
 
Monthly fraction of the number of meals served in a restaurant 
Period of use Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec 
4 seasons 
Summer holidays 
Winter holidays 
0,85 
0 
0,70 
0,78 
0,23 
1,79 
0,77 
0,27 
0,9 
0,73 
1,24 
1,79 
0,78 
1,43 
0,9 
1,05 
1,8 
1,34 
1,24 
2,05 
1,68 
1,25 
2,23 
1,68 
1,28 
1,72 
1,00 
1,28 
1,03 
0 
1,00 
0 
0 
0,99 
0 
0,22 
(Source EDF: Eau chaude électrique Résidentiel et Tertiaire – Mars 1987) 
 
 
 
 17 
2.3 Heating water with solar energy 
 
A domestic hot water supply system using solar energy is composed of 5 sub-systems: 
§ A collector sub-system, 
§ An energy transfer sub-system, 
§ A storage sub-system, 
§ A back-up energy sub-system, 
§ A distribution sub-system. 
 
The energy saved with reference to a conventional water heater, depends on the climate, 
the collector layout, the sizing and system design, as well as the components and their 
maintenance. 
Therefore, it is essential that the best relationship between the costs, the system size 
and the needs must be founded during the project design phase; this must also include 
the most effective design of all the sub-systems, so that: 
§ Solar energy collection and storage is optimal, 
§ Solar and back-up energy supplies are dissociated, 
§ Solar energy is used in priority, 
§ The back-up energy supply is only used as a complementary energy source. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Collection 
 
 
Transfer 
 
 
Storage 
 
 
Distribution 
 
Control Back-up 
 18 
2.3.1 Collection 
 
A solar hot water supply system is generally composed of flat plate solar collectors with 
fluid circulation, that transform the sun's emission of electro-magnetic radiation in to 
heat which is then passed on to a heat transfer fluid. 
 
2.3.1.1 Working principles 
 
A flat plate collector is essentially an absorbingsurface exposed to solar radiation. The 
absorbing surface transfers the heat produced by absorption and in heating, it emits 
thermal radiation at a higher wavelength. (Stefan-Boltzman's Law). 
 
 
 
 
 
 
 
 
 
 
Rayonnement solaire Rayonnement thermique 
 
 
 
 
 
Solar radiation Thermal radiation 
 
 
If the absorber is in direct contact with the ambient air, there will be important heat 
losses by convection as well as by radiation. A thermal balance will be established 
between the absorber and the ambient air. Therefore, little energy will be collected. 
 
In order to reduce the losses from the back of the collector, the absorber is placed in a 
box in which the inside surfaces are covered with thermal insulation (glass wool or 
expanded synthetic materials, for example). 
 
The thermal insulation in front of the absorber is composed of a material that is opaque 
to thermal radiation but transparent to solar radiation. 
 
Glass and certain synthetic materials are transparent to solar radiation and opaque for 
the infrared radiation. Therefore, they are used as transparent covers for solar collectors. 
 
In the case of a solar collector with a transparent cover: the cover absorbs the thermal 
radiation emitted by the absorber, heats up and radiates heat from both sides. As a 
rough estimation, half of the radiation is diffused to the outside and the other half is 
returned to the absorber. This is the greenhouse effect. 
 
 
Power emission 
5800 K 
l(m m) 
0,4 0,8 
Infra Red 
0,5 1 2 3 
UV Visible 
Power emission 
 
 
450 °C 
l(m m) 
1 5 8 10 
150 °C 
 19 
 
 
The glazing also limits heat loss by convection, as the thermal transfer between two 
surfaces that are separated by a layer of motionless air are mainly due to conduction and 
the motionless air makes effective thermal insulation. 
 
The quality of the insulation increased with the thickness of the air layer between the two 
surfaces, as long as the heat transfer is by conduction (2 to 3 cm). If the space between 
the two surfaces is too important, natural convection is set up and has the opposite 
effect. 
 
 
 
Section of a flat plate collector 
 
 
 
Another way of reducing collector heat loss is to add a selective coating to the absorber. 
This coating offers an absorption coefficient as high as possible for radiation in the solar 
spectrum (less than 2,5 mm) and an emission coefficient as low as possible in the 
infrared, that corresponds to the radiation of the absorber (wave lengths greater than 
2,5 mm). 
Such selective coatings are made by a chemical deposit on the absorbing surface. 
 
Finally vacuum tube collectors make it possible to reduce heat losses by convection, as 
the absorber is placed inside a glass tube from which the air has been withdrawn. 
1: transparent cover 
2: absorber fin 
3: tubular circuit 
4: thermal insulation 
5: casing 
 20 
 
 
Section through a vacuum tube collector 
 
 
 
2.3.1.2 Global energy assessment 
 
Whilst in permanent operation, the characteristics of a flat plate collector are obtained by 
the following global energy assessment equation. 
 
Qu = Qa _ Qp 
 
In which: 
§ Qu is the energy transferred to the heat transfer fluid, 
§ Qa is the solar energy absorbed, 
§ Qp is the energy corresponding to the thermal losses 
 
In order to estimate the energy that has been absorbed by the collector, one must make 
a theoretical distinction between the direct and the diffused radiation, by giving them 
appropriate transmission and absorption coefficients. 
 
However in practice, one considers the component of the incident global radiation that is 
perpendicular to the collector surface. 
 
In this case, the absorbed energy, in Watts, is given by the following equation: 
 
Qa = A . ts . as . G 
 
In which: 
- A is the surface area of the collector entry in m2, 
- ts and as are the average values of the transmission coefficient of the 
transparent cover and the absorption coefficient of the absorber for the 
complete solar spectrum, 
- G is the global input energy in W/m2 (entry area), measured in the collector 
plane. 
 
As a flat plate collector is relatively thin, the losses from the sides can be neglected in an 
approximate estimation, with only the losses from the front and back of the collector 
being taken in to consideration. These losses are expressed as follows: 
 
Qp = QAV + QAR 
 
When reduced to a unit of absorber surface, all the losses that are thermal flux from the 
collector to the outside, can be expressed in relation to the temperature difference that 
causes them, therefore: 
 
QAV / A = UAV (Tm-Ta) and QAR / A = UAR (Tm-Ta) 
 21 
 
 
That is: 
QA / A = U (Tm-Ta) 
 
Where: 
U = UAV + UAR 
UAV = Thermal loss coefficient from the front (W/m2. K) 
UAR = Thermal loss coefficient from the back (W/m2. K) 
Tm = Average absorber temperature 
Ta = Average ambient temperature 
 
 
The thermal balance at a given moment, between the solar energy received by the 
collector, the useful energy available and the thermal losses, make it possible to describe 
the instantaneous efficiency by means of the following equation: h = h0 – U (Tm-Ta) / G 
 
According to the principles accepted by the international (ISO) and European (CEN) 
standards, the efficiency of a flat plate collector can be described by three coefficients 
that are independent of the temperature: 
 
h = h0 – a1T* - a2 G (T*)2 
Where: 
§ h0 : optical conversion coefficient (%) 
§ a1 : thermal loss by conduction coefficient (W/m2.K) 
§ a2 : thermal loss by convection coefficient (W/m2.K2) 
 
The figure below shows the variations of the momentary efficiency in relation to the 
temperature T* = (Tm-Ta) / G for several different collector types. 
 
 
 
For values of T* less than 0,07: the variations can reasonably represent by means of a 
linear graph. 
 
The efficiency is then: 
 
 22 
h = h’ – a’T* 
 
In the French standard NF P50-501, the coefficients h’ and a’ are respectively called: 
§ Optical factor of the collector (B) 
§ Total thermal conductance losses (K) 
 
 
 
 
Global efficiency of a solar collector 
 
Useful thermal energy 
Optical losses 
Thermal losses 
Temperature difference between the collector and the ambience 
E
f
f
i
c
i
e
n
c
y
 
 23 
2.3.2 Energy transfer and storage 
 
2.3.2.1 Storage 
 
Storage of the collected energy makes it possible to compensate for the discontinuous 
nature of solar energy. The accumulation of stored energy is represented by a rise in 
temperature. 
 
In order to visualize the efficiency of a storage system, one must remember that the 
efficiency of a collector mainly depends on the average temperature of the fluid that goes 
through it, and therefore, on the temperature of the fluid coming from the storage 
system. One of the essential characteristics of an efficient storage system is the ability of 
supplying the collectors with a fluid that has a temperature as low as possible. 
 
The heat transfer from the collectors to the storage takes place in two different ways: 
§ By forced circulation using a pump operated by means of a control system, 
§ By natural circulation or thermosiphon. 
 
Systems operating by thermosiphon have an advantage in relation to the habitual 
systems using pumps, as they neither need control equipment for the solar gain, nor 
pumps for moving the heat transfer fluid. However, in practice, the installations in 
thermosiphon only involve certain types of individual water heaters and are exceptional 
in collective installations. 
 
Owing to the hydraulic problems in large size collector arraysand the architectural 
constraints caused by the necessity of placing the collectors below the storage tank, heat 
transfer fluid circulation by thermosiphon is generally not adapted to collective systems. 
 
 24 
2.3.2.2 Heat exchangers 
 
Throughout the French mainland, solar equipment must be protected from the risk of 
freezing. In most cases, the collectors are protected by the use of an anti-freeze fluid and 
this implies the use of a heat exchanger. 
 
There are two categories of heat exchanger: 
§ Heat exchangers integrated in to the storage tank. 
§ Heat exchangers outside the storage tank. 
 
In the case of a heat exchanger outside the storage tank, the exchange is made by 
forced convection. The surface area of the external heat exchanger is generally smaller 
than an integrated heat exchanger. 
 
 
 
 
 
 
 
 
 
 
 
 
 Flat plate external heat exchanger Integrated heat exchanger 
 
In any case, the efficiency of a heat exchanger does not depend on the temperature of 
the fluids but on the geometry of the exchanger and the heat flow. In practice, the 
efficiency of heat exchangers is from 0,6 to 0,8. 
 
Note: if the efficiency of the heat exchanger is mediocre, not only will the heat transfer 
be slight, but also the temperature of the fluid returning to the collectors will be high and 
the collector efficiency will be reduced. 
 
 
Simple heat exchanger sizing tool: calculation example (Source : Gret) 
 
 
 25 
When an exchanger is integrated in to a storage tank, it is placed in the lower part of the 
tank. This disposition makes it possible, if the exchange surface area is sufficiently 
important, to heat the volume of water in a homogeneous way, until the temperature in 
the lower part approaches 3 to 4 degrees from the temperature in the upper part of the 
tank. 
 
Cold water enters the bottom of the storage tank whenever hot water is used, avoiding 
the introduction of insufficiently heated water in to the upper part of the tank 
(Temperature stratification). 
 
This disposition also makes it possible to supply the solar collectors with a fluid that has a 
temperature as low as possible on leaving the heat exchanger, in relation to the hot 
water consumption and the temperature stratification. Furthermore, it limits the risk of 
heat loss by reverse flow in the collectors, in the case of an incorrect operation of the 
check valve. 
 
 26 
2.3.2.3 Primary circuit controls 
 
Principles 
 
The basic principles of collective hot water system controls are simple. 
One sensor is placed in the solar collector and another in the lower part of the hot water 
storage tank (at 1/ 9 of the height). 
As soon as the collector is a few degrees hotter than the storage tank, a pump starts up; 
when the temperatures are the same, the pump stops. A simple differential control is 
sufficient for these operations. 
 
The role of the control equipment is to order the transfer of the collected energy, only 
when the temperature of the heat transfer fluid in the collectors is higher than the water 
in the storage tank. 
 
For installations with a collector area of less than 40 m2, with short hydraulic circuits 
(less than 50m), and where the collectors have a relatively high inertia, the differential 
type of control system, based on the temperatures in the tank and the collectors, is used. 
 
 
 
 
TH
EF
.frtecsol
André-JOFFRE
Schéma n°02 31-05-2001
BP 434 - Technosud 
Perpignan Cédex
Tel : 04-68-68-16-40
Fax : 04-68-68-16-41
Schéma de principe eau 
chaude sanitaire
RD
Déflecteur
Manchette
Témoin
Stockage
Vers Circuit 
Appoint E.C.H
Batterie de
Capteurs solaires
VE
normalement ouverte
Vanne à boisseau sphérique 
normalement fermée
Vanne à boisseau sphérique 
Vanne d'équilibrage
Clapet anti retour
Soupape de sécurité
Purgeur d'air automatique
Manomètre
MA
Thermomètre
TH
Sonde de régulation
Vase d'expansion
RD Régulateur différentiel
Circulateur
TH
Raccordement vidange
RT
TH MA MA
MA
Differential control principles (Source Tecsol) 
 27 
 
In the case of larger systems (> 40 m2), one uses a double differential, with an 
additional sensor placed on the primary loop in the technical premises. This will start up 
the pump on the secondary circuit. In this case the system can be started up in two 
steps. The first step, where the primary loop is started up with a levelling of the 
temperatures in the solar collectors and the pipes, and a second step, where the 
secondary circuit is started up with energy transfer from the primary to the secondary 
loop. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Certain engineers prefer to use a photoelectric sensor instead of the second differential 
controller. Besides the difficulty in choosing a well adapted component (circuit closed 
when the light intensity is greater than a certain level and not the opposite as for lighting 
and a point of control higher than that which s used for lighting), we do not recommend 
this solution in as much as the operating time for the primary circuit is relatively 
important compared with that of the secondary circuit, and this is the cause of 
unnecessary electricity consumption. 
 
In both cases, the operation of the secondary pump must be dependant on the operation 
of the primary pump, in order to avoid the useless operation of the pump on the 
secondary circuit. 
 
 
 
 
 
 
 
 
Déflecteur
Manchette
Témoin
Stockage
Vers Circuit 
Appoint E.C.H
Batterie de
Capteurs solaires
VE
normalement ouverte
Vanne à boisseau sphérique 
normalement fermée
Vanne à boisseau sphérique 
Vanne d'équilibrage
Clapet anti retour
Soupape de sécurité
Purgeur d'air automatique
Manomètre
MA
Thermomètre
TH
Sonde de régulation
Vase d'expansion
RD Régulateur différentiel
Circulateur
TH
Raccordement vidange
TH
RD1 RD2
EF
.frtecsol
André-JOFFRE
Schéma n° 31-05-2001
BP 434 - Technosud 
Perpignan Cédex
Tel : 04-68-68-16-40
Fax : 04-68-68-16-41
Schéma de principe eau 
chaude sanitaire
05
T
H
TH
 
Differential controls: principles of the double differential (Source Tecsol) 
 28 
Regulating the differential controller 
 
This control method is simple and inexpensive. The good working order depends mainly 
on the temperature difference adjustments. 
 
Differential adjustments Consequences 
DT1 big 
DT2 big 
Starting up retarded in the morning. Solar energy unused. 
DT1 big 
DT2 small 
Starting up retarded in the morning. Late stop in the 
evening: loss of energy collected during the day. 
DT1 » DT2 Pumping phenomena 
 
 With: 
DT1 : adjustable temperature differential for controlled starting up 
DT2 : adjustable temperature differential for controlled stopping. 
 
The flow of the fluid in the collectors starts up when Tcollector > Tstorage + DT1 
The pumps are stopped when Tcollector < Tstorage + DT2 
The pumping phenomenon is undesirable as it causes ware of the pumps and lowers the 
efficiency of the system. It occurs very easily whenever there is little difference between 
the starting and stopping temperature differentials. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
The following figures can generally be used 
to ensure a good system operation: 
DT1 = 5 K – 8 K 
DT2 = 1 K – 3 K 
 
 
 
When the inertia of the primary loop is important (piping longer than 50 m), the primary 
circuit controls are completed by an action on the switch valve. When the temperature TC 
in the collectors is greater than the figure TC + DT1 of the water in the storage tank, the 
controller orders the pump to start up. 
The valve Vc is open to recycle the heat transfer fluid throughthe collectors (temperature 
homogenisation in the primary circuit). 
 
Although this control principle is based on an "all or nothing" switch valve, it reduces 
unnecessary pumping. Furthermore, the thermal performance of the installation is 
improved owing to the rapid warming up of the primary loop in the morning. 
 
 
 
 
 
 
 
 
R 
TC 
TB 
EF 
EC 
Differential controls 
R
TC
EC 
T1 
 29 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
§ If the temperature T1 in the primary 
circuit is greater than the temperature 
Tb of the water in the storage tank (T1 
> Tb + DT1), the controller orders the 
switch valve to open to the storage 
tank. There is heat transfer in the 
storage tank. 
§ If T1 < Tb + DT2, the switch valve closes 
the storage circuit. The heat transfer 
fluid flows to the collectors again. In 
this case, if Tc < Tb, the controller 
orders the pump to stop. 
 
 
 
 
When the system is equipped with an external heat exchanger, the flow of the domestic 
hot water in the secondary circuit of the heat exchanger requires the installation of a 
second pump. 
Generally, the system operation is ensured by the use of two differential controllers R1 
and R2. 
In order for the inertia of the primary circuit to be taken in to consideration (it can 
contain a large amount of fluid), one is advised to temporise the operation of the pump 
controlled by R1 so that frequent stops and starts of the system every day can be 
avoided. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
The pump on the primary circuit is 
operating (Tc > Tb + DT1): 
§ If (T1 > Tb + DT1), the controller R2 
orders the operation of the secondary 
circuit controls. 
§ If (Tc < Tb + DT2), the domestic hot 
water flow in the secondary circuit is 
stopped. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
For large-scale systems, and in order to avoid temperature measurement errors due to 
faulty collector array irrigation, one can use a solar radiation sensor instead of measuring 
the temperature in the collectors. 
R1 
TC 
TB 
EF 
EC 
T1 
R2 
Differential controls. External heat exchanger 
 30 
 
This variant is justified when the temperature of the fluid in the primary circuit needs to 
be homogenized owing to the size of the installation. However, it should only be used for 
systems with an external heat exchanger. Furthermore, this generally causes higher 
electricity consumption than the method described previously. 
 
When the solar radiation S is greater than the start up level S1, the controller R1 orders 
the primary circuit pump to start up. 
 
 
§ If (T1 > Tb + DT1), the controller R2 
orders the secondary circuit to operate 
(storage phase) 
§ If (T1 < Tb + DT2), the secondary circuit 
pump stops (bypass phase). When the 
solar radiation level is less than the 
threshold S2, the controller R1 orders 
the primary circuit pump to stop. 
 
 
 
R1 
S 
TB 
EF 
EC 
T1 
R2 
Differential controls. Radiation sensor 
 31 
 
2.3.3 Auxiliary heating 
 
Three different types of auxiliary heating are considered, according to the nature of the 
demand and the building configuration: 
§ Installations with centralised auxiliary heating and a circulation loop distribution 
system, with a total length of plumbing between the loop and each outlet of not more 
than an average 6m. 
§ Installations with a decentralised production and direct distribution or by means of a 
loop. The distribution is either direct (distance between the storage tank and the 
outlets less than 8 m) or by means of distribution loops for a group of outlets (the 
total length of the plumbing between the loop and each outlet should be less than 
6m). 
§ Installations with individual auxiliary heating and direct distribution, when the outlets 
are not more than 8m from the storage tank, in order to avoid thermal losses and 
wasting cold water. If this is not possible, individual auxiliary heaters are used to 
maintain a circulation loop at a fixed temperature. 
 
 
In the case of installations with centralised auxiliary heating, the domestic hot water 
redistribution loop should be designed so that the auxiliary heater compensates the 
thermal losses. 
When the auxiliaries are individual, a local back up should keep the domestic loop at the 
required temperature. 
 
 
 
 
 
 
 
Example of an installation with a separate auxiliary heater and a domestic hot water supply loop 
R1 
TC 
TB 
EF 
EC = 60 °C 
T1 
R2 
EC = 50 °C 
 32 
3. The project procedure 
 
3.1 Preliminary studies 
 
The preliminary studies prior to designing a solar hot water supply system are aimed at 
estimating the potential interest of a future installation in relation to the demand for hot 
water (quantity and regularity throughout the year) and the existence of technical or 
architectural constraints, by: 
- Sizing the installation with reference to the different constraints, 
- Estimating the cost, 
- Estimating the provisional savings. 
3.1.1 Estimating the needs for hot water 
 
The analysis of the needs is an indispensable part of the preliminary studies that comes 
before the choice of production equipment. 
When the needs are estimated correctly, sizing and calculation tools can be used to 
estimate the provisional performance. 
 
The Guaranteed Solar Results contract (GRS), that makes a commitment on the 
companies that have designed and built the system, is based on the provisional energy 
production, which is calculated from the preliminary hot water demand estimation. 
Therefore, it is essential that the needs are known as precisely as possible. 
However, the real consumption hypotheses are frequently unknown. In this case, the 
known statistics used to estimate the typical consumption of the type of establishment 
concerned are insufficient, and real in situ measurements are recommended before 
designing the project, particularly in the case of large systems in the tertiary or health 
sectors, where important variations in the demand have been found. 
 
In the case where one knows, by means of a volume flow meter, for example, the 
amount of hot water Vecs (m3/day) supplied to the outlet points, one can determine the 
daily energy needs Becs in kWh/day, by means of the equation: 
 
Becs = 1,16 . Vecs . DD T 
 
In which DT is the average temperature difference between the hot water distributed to 
the users and the cold water supply. 
 
When the measured domestic hot water consumption data is not available, the daily 
demand can be estimated on the basis of the amounts of hot water required for the 
principal conventional needs, taking in to consideration the fact that not all the needs are 
at the same time (simultaneity coefficient). 
 
 
 
 33 
3.1.2 Solar radiation on the site 
 
For a given site, the amount of energy received by a collector depends on the site's 
energy exposure, and the installation layout. 
Data concerning solar radiation can be obtained from meteorological stations throughout 
the country or from Climatic data surveys: 
§ Sunshine duration and relative sunshine duration in France, from 1951-1970, Garnier 
M., Monography n°105, Météorologie Nationale, 1978. (Durée et fraction d'insolation 
en France, période 1951-1970 avec longues séries de mesures). 
§ Solar Radiation Atlas for France, Tricaud J.F., Pyc Edition, 1979. (Atlas Energétique du 
rayonnement solaire pour la France). 
§ The solar resource in France, Statistical data, 1971-1980, Météorologie Nationale, 
1984. (Le gisement solaire en France: recueil de données statistiques, 1971-1980). 
§ The solar resource in France, Daily data, 1971-1980, Météorologie Nationale, 1984. 
(Le gisement solaire en France: recueil de données quotidiennes,1971-1980). 
§ Building construction climatic atlas, Chemery L., Duchene-Marullaz P. - CSTB, 1987. 
(Atlas climatique de la construction). 
§ European solar radiation atlas (Atlas européen du Rayonnement Solaire).Volume II : 
Inclined surfaces (Surfaces inclinées). W.Palz, Commission des Communautés 
Européennes; Direction Générale Science, Recherche et Développement. 
§ Regional atlas of daily insolation frequency (Atlas régionaux des fréquences de 
l'insolation journalière), Programme PIRDES-CNRS. 
 
 
Or from the databases that are available on line through the Internet: 
§ Satel-Light, The European daylight and solar radiation database 
(http://www.satel-light.com), 
§ NASA. Surface meteorology and Solar Energy Data Set 
(http://eosweb.larc.nasa.gov/sse/), 
§ Solar Radiation and Radiation Balance Data -The World Network- 
(http://wrdcmgo.nrel.gov/html/get_data-ap.html). 
 
 34 
3.1.3 Collector layout study 
 
3.1.3.1 Adaptation to the climatic conditions 
 
In practice, solar collectors should be installed in such a way that the periods in which all 
or a part of the array is shaded by neighbouring obstacles, is as short as possible. 
This condition is considered to be fulfilled if, during a sunny day, the complete collector 
surface area is exposed to direct solar radiation for at least 4 hours during the month of 
December and during at least 8 hours during the month of June. In metropolitan France, 
the normal slant of solar collectors, which are used throughout the year, is generally 
between 30 and 45 degrees from the horizontal. However, on sloping roofs, the collectors 
are generally installed in the plane of the roof, for aesthetic reasons and also to resist the 
climatic loads (wind and snow). 
The COMMBât software (developed and distributed by the CSTB) stipulates the general 
regulations that are applied in every French locality. It provides the general 
characteristics that are applied to buildings concerning: snow loads, wind pressure, wind-
rain concomitance areas, climatic zones, basic design temperatures, etc. It outlines the 
design and construction rules according to the locality and in relation to the site. 
 35 
3.1.3.2 Influence of shading and obstacles 
 
The influence of shading caused by distance obstacles can be estimated in a simple way 
by measuring the angles of the projected shadow on a solar diagram or by using a 
calculation method to establish the solar radiation that will be available on a sunny day 
throughout the year, for example: 
§ The solar diagrams from the CSTB, 
§ The polar diagrams from the Group ABC, School of architecture in Marseilles. 
 
 
 
The solar diagram shown above was established for an observation point situated on the 
line of latitude 0° North. It represents the sun's apparent movement across the sky at 
different dates in the year. The concentric circles represent the sun's height above the 
horizon. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Solar diagram for 
an observation 
point situated on 
the line of latitude 
45° North 45 ° 
South 
 36 
 
 
 
 
 
A few examples of solar diagrams (latitude 44°): 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
A: June 
B: May-July 
C: April-August 
D: March-September 
E: February-October 
F: January-November 
G: December 
 
 37 
 
 
 
 
 
The solar factor f takes the shadows projected on to the collectors in to consideration. 
When there is no significant shading on the collectors, the solar factor equals one. 
 
- Case study 1 : The shading is mainly caused by distant obstacles. 
The annual value of the solar factor f is in relation to the average height of the 
obstacles in front of the collectors, above the horizon. 
The following graph was established for the case where the obstacles in front 
of the collectors have a constant height above the horizon. It can be applied to 
real conditions if the variations in the height are not very important. 
 
 
 
 
§ Case study 2 : Shading is mainly caused by a straight sided, close or distant obstacle 
(for which the upper edge is approximately parallel to the upper edge of the 
collectors, and sufficiently long for the border effect to be neglected). 
 
For example: collective installations with rows of collectors that project 
shadows on each other, or in the case of building that shades the collectors. 
 
In this case the solar factor is given in the table below, in relation to the two 
angles aa and bb expressed in degrees (obstacles infinitely long and collectors 
facing South). 
 
 
 
bb 
aa
 
-10 -5 0 5 10 15 20 25 30 
0 1 1 1 0,99 0,97 0,92 0,85 0,76 0,67 
15 1 1 1 0,98 0,95 0,90 0,84 0,75 0,67 
30 1 0,99 0,97 0,95 0,92 0,88 0,82 0,74 0,67 
45 0,98 0,97 0,96 0,93 0,90 0,85 0,79 0,72 0,66 
 
 
 
 
 
 
 38 
The angles aa and bb can be determined in relation to the lengths shown in the figure 
below: 
 
 
 
 
 
 
 
 
 
 
 
 
 
tg b = [L.h – l.(h-H)] / [l.L - h.(h-H) ] 
tg a = (2h-H) / (L+l) 
 
 
 
 
 
 
 
When the obstacle is an array of collectors at the same height and the same slant as the 
collectors under consideration, the calculation of the angles aa and bb is simplified: 
 
 
 
 
 
 
tg b = h / l 
tg a = h / (L+l) 
 
 
 
 
The minimal distance l between two rows of collectors can be obtained from the following 
equation: 
 
l/h = [(l / tg a) – (l / tgi)] /2 
 
When there are several rows of collectors, the coefficient f can be different for each row. 
An average of these values, weighted to the surface area of each row, can then be 
applied. 
 
aa
bb 
H
h
l 
L
aa
bb 
i 
h
l 
L
 39 
3.1.3.3 Architectural integration 
 
Architectural harmony is an important point in the successful integration of solar 
components in a building. Even if the most frequent solution up to now, is the installation 
of solar collectors on independent structural support, on sloping or on flat roofs 
throughout Europe, collector manufacturers, architects and their clients work on 
improving the integration of solar collectors in to the environment. 
Positive solutions for the integration of collectors in to roofs are well known, particularly 
in Northern Europe, but they do not provide an answer to all the different situations. 
Collectors in the walls are only adapted to space heating. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
It should be remembered that thermal solar collectors are submitted to Technical notice 
procedures that fix the conditions in which they can be used and that the installation of 
solar equipment requires obtaining a building permit. The client should make a request at 
the local municipal planning office or the authorities concerned. 
 
The general rules for collector installation approved by the Specialised groups n°14 and 5 
of the Commission responsible for formulating the Technical notices, within the frame 
work of the "Recommendations for installing solar collectors" ("Recommandations 
Générales de mise en œuvre des capteurs solaires" Cahiers du CSTB n° 1612 et 1613) 
only concern the installation of collectors in new buildings. 
 
 
Independently supported 
collectors on a flat roof 
 
Roof integrated solar 
collectors (source: AIE) 
 40 
 
 
Example of the installation of solar collectors on a flat roof 
 
 
 
 
Example of the installation of solar collectors on a sloping roof 
 41 
3.1.3.4 Obstacles and accessibility 
 
When installing solar collectors on existing roofs, the different layout plans defined below 
can be appliedif certain additional factors are considered, notably concerning the position 
of the roof structure, the roofing material, the thermal insulation and waterproofing, as 
well as draining the rain water. 
 
In any case, the stability of the collector array should be studied in relation to its own 
weight and to the effect of the climatic load, with respect to the regulations in vigour: 
§ Regulations defining the effects of snow and wind on building structures (Regulations 
NV 65, Regulations NV 84), 
§ Regulations for calculating and building metal structures (Regulations CM 66), 
§ Regulations for calculating and designing timber roof structures (Regulations CB 71), 
§ DTU n° 65.12 "Installing flat plate liquid flow solar collectors for heating and hot 
water supply (Réalisation des installations de capteurs solaires plans à circulation de 
liquide pour le chauffage et la production d'eau chaude sanitaire)", Cahier du CSTB 
2204, N° 285, December 1987. 
§ Determination of the forces applied to solar collectors and their glazing due to climatic 
loads (Cahier du CSTB 1611, N° 204, November 1979). 
 
 
 42 
 
 
 
 
 
 
 
Diagrams showing the connection between structural elements on a roof or the 
anchorage of braces (Source: CSTB) 
 
 
 
 
 
 43 
 
 
 
 
Examples of pipe penetration (Source CSTB) 
 
 
 44 
3.1.4 Collector connections 
Solar collectors are included in the Technical notice procedures concerning non-traditional 
heating and cooling equipment. 
A solar system is composed of collectors that are of the same type and make. If this is 
not the case, or if one of the collectors is replaced, the absorbers should be made of the 
same materials in order to avoid metallic couples that could be the source of corrosion 
inside the collectors. 
The collectors should be placed on supports: 
§ In such a way that the flatness of the collectors is respected; the assembly should 
never warp the collectors, 
§ In such a way that the orifice for draining condensation is situated on the lowest point 
of the collector, 
§ That are capable of withstanding the extreme climatic loads (wind and snow). 
 
All the collectors should have similar physical characteristics, notably concerning 
circulation pressure loss. This is particularly important, as it is the cause of hydraulic 
balance difficulties in the collector array. In any case, the recommendations in the 
manufacturers technical notice should always be respected, particularly concerning the 
connection of the collectors together and the problems due to expansion. 
One of the frequent causes of differences between the measured and the estimated 
thermal performance of a solar system is due to a lack of balance in the collector array. 
The most common technique for ensuring that the flow rates are well balanced is to 
adjust the valves on the installation. This empiric method offers uncertain results and 
cannot correct design errors concerning the hydraulic connection between the collectors. 
A few hydraulic connection layout plans make it possible to avoid the most frequent 
design errors. They have been established from calculation software and on site 
experiments aimed at estimating the real solar system performance and at optimising 
the hydraulic circuits. (A. Lebru. "Hydraulic behaviour and real thermal solar system 
performance estimation" Comportement hydraulique et évaluation des performances 
thermiques réelles des champs de capteurs. Document CSTB- Ref.MPE/411 – May 1985). 
These recommendations are not exhaustive. They are useful in the case of certain 
connection schemes, and they ensure the best guarantee of good working order for the 
most frequently used fluid flow rates (from 40 to 70 l/h.m2). 
As a general rule, the Tickelman loop can be used for the links between the collectors 
and the arrays. However, the diameter of the manifold must be adapted to the number of 
collectors and their flow pressure losses. In order to ensure a certain similarity in the 
flow rates through the different collectors, the following ratio: 
Pressure losses in the manifolds/Pressure losses in the collectors 
should be as low as possible, which means that the ratio: 
Internal manifold diameter/internal hydraulic circuit diameter through the collectors 
Should be as low as possible (ratio between 1,6 and 3,3). 
Other types of configuration an be considered: 
§ Parallel connections, 
§ Parallel series connections. 
In any case, one should avoid connecting more than 5 or 6 collectors in the same array. 
 
 45 
 
 
 
 
Tickelman loop 
connection 
 
 
Tickelman loop 
connection 
 
Connection in series 
(flow rate > 70l/h.m2 
and N<5) 
 
Connection in parallel 
(N<5) 
The flow rate is the 
same in each collector 
if: 
Di coll / Di capt 
minimal 
 
Arrays in parallel series 
 
 
 
 
 
 46 
3.1.5 Plumbing 
 
The choice of the piping and components in the hydraulic circuits should be in conformity 
with the codes of practice for plumbing (standard NFP 41-201 and DTU in the 60 series), 
and the sanitary regulations in vigor. 
 
Particularly: 
§ Galvanised steel pipes should not be used for water with a temperature higher than 
60°C. Moreover, they should not be placed in a circuit after copper, brass or bronze 
piping, 
§ The use of untreated pipes of with an internal coating of surface treatment is 
authorised, on condition that the materials figure in the list published by the High 
council for Public hygiene in France (Official Journal Brochure n° 12227) and that they 
have no effect on the water supply. 
 
In practice, and unless there is a counter indication in the specifications joined in annex 
to the technical notice, the following materials are considered to be satisfactory: 
§ Copper pipes in conformity with the standard NF A 51-120, with a thickness equal to 
or greater than 0,8 mm, 
§ Steel pipes in conformity with the standards NF A 49-115, NF A 49-11, NF A 49-112, 
NF A 49-160, NF A 49-141, NF A 49-142, NF A 49-145, NF A 49-150, NF A 49-120, 
NF A 49-250, 
§ Non metallic materials with a technical notice that states the compatibility of the 
material and the heat transfer fluid (resistance to heat, pressure and chemical 
compatibility …). 
 
Use not recommended for water based fluids except in 
particular conditions 
Type of absorber circuit 
Non ventilated circuit Ventilated circuit 
Black steel 
Prohibited for sanitary use 
except when treated in a 
recognised way 
 
 
pH < 5* or pH > 12* 
 
pH < 9* or pH > 12* 
 
Anti-freeze protection by 
emptying 
 
Galvanised steel 
Phohibited for use in heating 
 
pH < 7* or pH > 12* 
Presence of copper in the 
circuit upstream 
pH < 7* or pH > 12* 
T > 55 °C 
 
Stainless steel Non compatible fluid Non compatible fluid 
Aluminium pH < 5* or pH > 8* 
 
Presence of copper or 
copper alloys (series 2000) 
or tin base (series 7000) 
pH < 5* ou pH > 8* 
 
Presence of copper or 
copper alloys (series 2000) 
or tin base (series 7000) 
Copper Presence of ammonia or by- 
products 
Presence of ammonia or by- 
products 
High sulphate and chloride 
content 
Anti-freeze by emptying (pH 
< 5) 
* Except when an anti corrosion product is used 
 
 
 47 
Sizing the primary circuit leads to the pipe diameter calculations with reference to the 
other factors that concern the flow of fluid: 
- The flow rate, 
- The volume and the viscosity. 
 
The primary circuit pipes should have a diameter that is sufficiently large for ensuring 
that the heat transfer fluid flows at the recommended rate, generally from 40 to 70 l/h 
per m2 of collector area, with a flow rate less than or equal to 1m/s. 
Severaldiameters are possible. However, it should be noted that: 
§ If the pipe diameter is reduced, the pressure losses rise, which leads to an increase in 
the power needed (pumps), 
§ If the diameter is increased, the pressure and the power needed is reduced, but the 
heat losses rise and the installation costs are more important. 
 
The economic factors should be considered when making the choice of diameter: 
§ Installation costs (materials, work time), 
§ Insulation costs (insulating materials, work time), 
§ Service costs (power consumption, maintenance, repairs…), 
§ Pay back time and the influence on the investment and the profitability of the project. 
 
Therefore, every project is a special case that should be considered attentively. It should 
be noted that: 
§ The economic diameter does not depend on the length of the circuit or the height rise 
of the heat transfer fluid, 
§ The unit price of the power and the materials used effect the profitability calculations 
in a similar way, in spite of the economic variations, 
§ The different factors involved in the choice of the most economic diameter has a 
square root relationship on the calculations, so the variations in the final result are 
slower than the variations of the different factors. 
 
 
 
 
 
 
 48 
3.1.6 Storage volume and back up 
 
At the preliminary study stage (feasibility study or pre-diagnosis) the possibility of 
passing the hydraulic circuit from the collector array to the storage tanks should be 
considered (presence of technical shafts or reserved spaces, building structure…), with 
reference to the diameter of the pipes, the need for maintenance access and the possible 
noise that could be caused by the flow of the fluid. 
A good setup consists in placing the tank or tanks close to both the collector array and 
the back-up supply, so that the length of pipes is as short as possible and the heat losses 
reduced. When this double condition cannot be fulfilled, the solar storage tank should be 
placed close to the back-up supply. 
The total volume of water stored at 60 °C in the back-up tank should be as least as much 
as the needs, at that temperature, during a day without sunshine. 
 
However, the storage volume should be increased with relation to these needs, if: 
§ The periods during which the hot water is used are separated from the periods, in 
which the back up is in operation (late use, absence of time relays…), 
§ The distribution is made without a re-circulation loop where the needs are frequent 
and the outlets are both numerous and distant. 
The unit volume of the back-up tanks should be chosen, when possible, from the range 
of equipment on the market, with a size less or equal to 5000 litres, considering the 
available space. Therefore, the required number of tanks will be chosen to make the 
installation, maintenance and replacement as simple as possible. Unless specified, the 
maximum number of tanks should be 2 or 3. If there volumes are different, there 
relationship should not be greater than 1 to 2. 
 
Example of an installation with a separate back up and DHW re-circulation loop 
 
 
L’importance des installations peut conduire à adopter des dispositions spécifiques telles 
que le bouclage de l’eau chaude distribuée. 
D : Photo-electric light meter 
R : Temperature differential controller 
Solar
tank 
Back 
up 
tank 
Flow meter 
Loop return 
 
 
 
 
High temperature 
security mixer tap 
 49 
The use of a distribution loop increases the storage losses by about 30 to 50% and 
reduces the solar fraction by about 10%. 
In certain cases, the losses from the circulation loop can be compensated by an electric 
immersion heater or by a re-heater placed at the extremity of the loop. This is the case 
of a system with a centralised back up, when the auxiliary heater is supplied by 
electricity during the off peak hours. 
In all events and to satisfy the sanitary regulation requirements, the sanitary hot water 
re-circulation loop should be designed so that the water is re-heated by the back up. 
The storage tanks and the heat exchangers should be thermally insulated (By-law 23rd 
June 1978). 
When the tanks are manufactured, the standard NF C 73-221 fixes the authorized heat 
losses. 
When the insulation work is carried out on the site, the characteristics of the thermal 
insulation should be such that the tank's heat loss is less than 2W /m3.K. 
The insulation should be fixed so that the equipment can be moved or removed for 
maintenance. If it is planned to remove the insulation, it should be possible to reinstall it 
without having to add additional materials or finishing. 
 
The storage tanks 
 
Except for specific cases (collector area < 20 to 30 m2), the storage tanks are the basic 
"DHW buffer tank" without an internal heat exchanger. Flat plate heat exchangers are 
recommended for the transfer of energy from the collectors to the storage tanks, for both 
economic and efficiency reasons. 
 
In certain applications, or when the storage tanks are used for low temperatures, tanks 
with internal heat exchangers are used in order to avoid the risk of freezing in the pipes 
and the heat exchanger. In these cases, the hot and cold water pipes should be well 
insulated. 
Particular attention should be given to the temperature resistance of domestic hot water 
storage tanks. Certain products are only guaranteed if the stock is less or equal to 60° C. 
Solar tanks must resist a minimum temperature of 80° C. 
Besides considerations concerning the quality of the materials in contact with domestic 
water supply (notably metallised coatings), there are no specific specifications linked to 
the use of solar energy. 
 
 
 
 
 
 
 50 
In order to encourage temperature stratification in the solar storage, the tank should be 
designed to optimise the energy performance of the installation. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Solar hot water tank: Placement of specific openings (Source Clipsol). The non-
respect of this geometry can be the cause of a drop in performance of up to 10%. 
H : Height of the tank 
100 : sleeve 40/49 
H/9 : sleeve 15/21 
H/3 : sleeve 40/49 
Sleeve for the 
protection anode 
9H/10 : sleeve 15/21 
Solar hot water tank 
Placement of specific openings 
 51 
 
 
 
 
In certain cases, due to a lack of space in the technical premises, one can be obliged to 
install several storage tanks in order to provide the required volume. In this case, the 
tanks should be places in series and supplied with cold (or warmed) water in a given 
direction, opposed to the flow through the heat exchanger. 
 
Vers Circuit 
Appoint E.C.HBatterie deCapteurs solaires
VE
normalement ouverte
Vanne à boisseau sphérique 
normalement fermée
Vanne à boisseau sphérique 
Vanne d'équilibrage
Clapet anti retour
Soupape de sécurité
Purgeur d'air automatique
Manomètre
MA
M
anchette
T
ém
oin
E
F
B1 B2
Raccordement vidange
.frtecsol
André-JOFFRE
Schéma n° 06 31-05-2001
BP 434 - Technosud 
Perpignan Cédex
Tel : 04-68-68-16-40
Fax : 04-68-68-16-41
Schéma de principe eau 
chaude sanitaire
Thermomètre
TH
Sonde de régulation
Vase d'expansionVE
RD Régulateur différentiel
Circulateur
MA
MA MATH
TH
RD1 RD2
TH
TH
RD3
MA
MA
Layout diagram: hot water storage with two tanks (Source Tecsol) 
 
 52 
 
 52 
3.2 Detailed study 
3.2.1 Sizing the solar installation 
3.2.1.1 Sizing principles 
 
It is not reasonable to expect the sun to provide 100% of the domestic hot water needs, 
on the French mainland. 
So a solarsystem must be linked to an auxiliary back-up another energy source: gas, 
electricity… 
Sizing the back up is aimed at ensuring: 
§ The hot water service at all times. This problem requires a good knowledge of the 
demand, 
§ The best possible partition between the solar and the auxiliary supply. 
 
It's a comp licated problem that requires the use of specific calculation software in order 
to supply a maximum amount of solar energy at a competitive price. 
Effectively, the cost of the kWh supplied depends mainly on two parameters: 
§ The cost of the of collector area per m2, 
§ The productivity of the system (annual production of the collector area per m2). 
 
The cost of the system can be estimated by using a simple formula. It is based on the 
collector area, in a virtually linear way. 
However, the productivity of the collectors cannot be defined simply. It drops when the 
collector area rises: the last square metre of collector area produces less than the first. 
Consequently, all the increase in surface area over an optimal system size leads to an 
increase in the cost of the kWh produced. 
If one or another of the system components are badly sized (storage, heat exchanger, 
plumbing, controls) the productivity will be penalized. 
The sizing method shown here is based on a simple and reliable system performance 
calculation method for solar hot water production; it takes the different phenomena 
described above into consideration. 
 System sizing method 
 
- Required data collection. 
- Definition of the operating principles 
- Pre-sizing of the principal components 
- Optimising the sizing of the solar equipment by using the estimated 
results of the different variants 
- Finalising the sizing of all the components 
 53 
3.2.1.2 Example 
 
This example is intended to show the influence of sizing the collector area and other 
components, by studying the results obtained from a solar systems with different sizes, 
situated in Perpignan and intended to supply the following constant amount of hot water: 
2 000 litres/day at 45°C. All the calculations have been made with the SOLO method. 
First of all, a reference system is defined that supplies 80% of the needs, 20% being 
supplied by an auxiliary back-up heater. This system is composed of 40 m2 of collectors 
and a 2000 litres storage tank. It produces 20,2 MWh/year for the total demand of 25 
MWh/year. The average productivity of the collectors is relatively low: 504 kWh/m2 
because of the high solar fraction. 
Then these reference figures concerning the collector area and the storage volume are 
modified. This makes it possible to draw the two graphs below. 
 
9 0 0 
8 0 0 
7 0 0 
6 0 0 
5 0 0 
4 0 0 
3 0 0 
2 0 0 
1 0 0 
0 
2 0 0 1 6 0 1 2 0 8 0 4 0 0 
C o l l e c t o r a r e a ( m 2 ) 
C o n s u m p t i o n : 2 m 3 / d a y , s t o r a g e : 2 m 3 
2 m 3 
P r o d u c t i v i t y M a r g i n a l c o s t 
 
 
5 2 0 
5 0 0 
4 8 0 
4 6 0 
4 4 0 
4 2 0 
4 0 0 
3 8 0 
4 . 0 3 . 0 2 . 0 1 . 0 0 . 0 
S t o r a g e v o l u m e ( m 3 ) 
C o n s u m p t i o n : 2 m 3 / d a y , c o l l e c t o r s : 4 0 m 2 
P r o d u c t i v i t y ( k W h / m 2 / y e a r ) 
 
Productivity variations with relation to the size of the principal components 
The collectors' productivity drops when the area increases and the marginal productivity 
(that of the last collector) soon becomes very slight: 
§ The 1s t m2 supplies about 850 kWh/year; the 20th m2 supplies 500 kWh/year, 
 54 
§ The 40th m2 supplies about 150 kWh/year; le 80th m2
 
supplies less than 50 kWh/year, 
§ Over 100 m2, the collectors produce virtually nothing. 
 
The variations in the size of the storage tank have little influence over 2.000 litres. 
However, if the storage volume is too small, the productivity drops: 
§ A volume of 2.000 litres permits a productivity of about 500 kWh/year, 
§ A volume of 1.000 litres permits a productivity of about 475 kWh/year (-5%), 
§ A volume of 500 litres permits a productivity of about 400 kWh/year (-25%), 
 
The figures will obviously be different in other situations, but the tendencies will be the 
same. 
 55 
3.2.2 Solar system performance estimation 
3.2.2.1 Estimation of the energy supplied instantaneously by a solar collector 
 
The fundamental equation 
The energy supplied instantaneously by a solar collector can be defined by a simple 
equation in which the collector is characterised by its surface area and two coefficients. 
This equation can be in different forms according to the reference temperature of the 
fluid. 
In France, the average temperature of the fluid in the collectors is used. For international 
standards, the collector inlet temperature of the fluid is preferred. 
Therefore, the equation can be written in two ways: 
 Pu = S (B I - K (Tfm-Te)) or: Pu = S (Fta I - FrUl(Tfe-Te)) 
Where: 
Pu : useful power (W/m2) 
I : radiation available on the collector surface (W/m2) 
Te : ambient temperature (°C) 
Tfe : collector fluid inlet temperature (°C) 
Tfm : average temperature of the fluid in the collectors (°C) 
S : collector surface area (m2) 
B, Fta : coefficient of the collector gains (-) 
K, FrUl : coefficient of the collector losses (W/m2/°C) 
 
Definition of the coefficients 
In order to be precise, the coefficients are variables that depend on the conditions at a 
given moment. In practice, a given collector can be characterised by a unique couple (B, 
K) or (Fta, FrUl) established by measurements in standardized conditions. The 
performance of a solar system, using this type of collector, can be estimated from the 
standard figures that are mentioned, for example, in the technical notices for the 
collectors sold in France. 
The value of the coefficient B for flat plate collectors is generally between 0,7 and 0,8, 
and between 0,5 and 0,8 for vacuum tube collectors (normal incidence). 
The value of the coefficient K for flat plate collectors is generally between 4 and 10 
W/m2/°C and between 1,5 and 3 W/m2/°C for vacuum tube collectors. 
The value of the coefficients Fta and FrUl are slightly lower than those for B and K. One 
can easily calculate the value of one from the other. Here we use B and K, as they are 
the only coefficients recognised in France, while waiting for the application of the 
European standards. 
 
 
 
 
 
 
 56 
 
Collector surface area 
There are several ways of defining the surface area of a collector. The terms the most 
commonly used are "overall surface area" and "useful surface area": 
§ Overall (Sht) is the area of the collector casing, 
§ Useful, is smaller, it's the solar radiation inlet area (glazing) Se. 
 
The overall surface area makes it possible to estimate the space requirements whereas 
the useful surface area is more significant in terms of thermal efficiency. 
The difference between these two figures is relatively small for flat plate collectors (5% 
to 10%), and more important for vacuum tube collectors (10% to 20%). 
The specifications given by the manufacturers refer to either one or the other or both. 
The technical notice from the CSTB gives both the entry the overall area. In practice, one 
can use either one if all the specifications refer to a surface area defined in the same 
way. If this is not the case, there is a risk of significant errors in calculating the technical 
and financial performance. Particularly: 
§ In the performance calculations, the collector specifications (coefficients B and K) 
used have been established for a given reference surface area,1 
§ In the economic calculations, the unit costs (per m2) are defined in relation to the 
same given reference surface area. 
 
For example: if we consider a collector