Solar Energy Conversion

SOLAR ENERGY CONVERSION
1. Introduction and basics of solar energy
The energy sources of the future are being discussed concerning the global climate change caused
by the human activities mostly connected with exploitation of fossil fuels (oil, coal, gas). On the
other hand the reserves of these fossil energy source also called conventional are going to their
end. From this point of view only the renewable energy sources satisfy the ecological
requirements and environment safety.
The low of energy conservation states that the energy cannot be created or destroyed, only
conversion from one form to another is possible. The energy can be transformed and it is possible
to exit in different forms. So the law can be applied to the solar energy conversion. The solar
energy can be transformed into heat of a body, only if the body is exposed to sun beams. So the
solar energy also is form of energy in accordance with the energy conservation law. The act of
conversion is the contact of solar radiation with a body surface.
Simply the contact and transformation of the solar energy may be presented so, fig.1 :

receiver
(collector)

solar radiation

conversion

heat

Fig.1. Principle of the thermal use of solar energy (Image: Fraunhofer ISE, Freiburg, Germany;
Solarpraxis AG, Berlin, Germany).
1

There are also other conversion processes - chemical energy into thermal, thermal into
mechanical (internal combustion engines), electrical to thermal, etc.etc. In the case of solar
energy transformation, the characteristics of the body receiving the solar energy are very
important. The body is so called “receiver” and its properties are determining the quantity of the
energy of transformation. In Table 1 some characteristics of the materials when they are in
process of interaction with solar radiation are given :
receiver

conversion to

energy form

black metal

heat

thermal energy

biological plants

biomass

chemical energy

solar cell

electricity

electrical energy

solar collector

heat

thermal energy

Table 1: Possibilities for using solar energy.
Thermal Radiation
The theory of light as a particle flux is developed by Max Planck and Albert Einstein. This
concept differs from the classical wave theory. Their interpretation is based on the assumption
that the radiation is a quantised flux. Also Niels Bohr has formulated the complementary
principle. It states that microscopic physical systems can only be described by mutually exclusive
models. At microscopic level, the structure is different to those at macroscopic. So, the postulated
“wave packets” called photons (in Greek light) are carrying the following characteristics :


They are invisible and can be created or annihilated.



The photons have no mass.



Each photon can transport only a defined amount of energy – light quantum.



If the photons of the same frequency have same energy, the intensity of radiation is
determined by the photons number.

In spite of mass lack, the photons are acting some way like material particles. It is accepted to
thing of the photons in a way of “wave packet”, connected with the frequency and energy
2

quantity E = h . , where h is the Plank constant and “ " is the frequency. After the contact of
the photons with the material body, a transfer of energy and momentum is transferred. Every
photon has wavelength, connected with its energy. The shorter wavelength of the photon is, the
higher energy it has. So the radiation of the Sun is flux of photons with different energy and
wavelength. The distribution of photons in dependent of their wavelength is called spectrum. The
human eyes can fill only a part of this spectrum, which we accept as visible, fig.2. The photons
can also create a visible colour effect in our eyes, fig.3 and the colours correspond to wavelength
or energy.

visible

spectral irradiance (W m -2 )

ultraviolet

infrared

Spectrum AM 0

Spectrum AM 1,5

high energy

low energy

wavelenghth (m)

Fig.2. Solar radiation spectrum (Image: Fraunhofer ISE, Freiburg, Germany; Solarpraxis AG,
Berlin, Germany).

violet

blue

green

yellow

red

wavelength [nm]

3

Fig.3. Colours in the visible range of the solar spectrum (Image: Fraunhofer ISE, Freiburg,
Germany; Solarpraxis AG, Berlin, Germany).
Heat
The heat is a form of energy, caused by interactions of the atoms. Even in the solids, the atoms
are not still, oscillating around their position. It is accepted the temperature is a measure of
kinetic energy of the atoms and the movement of these microscopic particles is in dependence of
their temperature. The thermodynamic postulates when heat is supplied to a body, the atomic
motion of the bodied increases and its internal energy rises.
Transfer of energy by thermal radiation to a body
The theory says a photon can transfer its energy completely to the atom. After the energy transfer,
the photon does not exist anymore. All of the processes of interaction between photons and
bodies surface can be explained with 3 simple conditions, fig.4 and consideration the
macroscopic body :


Absorption – the photon is absorbed by the body.



Reflection – the photon is reflected by the surface.



Transmission – the photon passes through the body.

The theory is dividing the bodies into 3 parts and also the combinations between them, fig.4 :


“Absolute black body” – all the photons are absorbed. The Sun has the characteristics of
an “absolute black body”



“Ideal reflector” – all the photons are reflected from the surface.



“Absolute transparent body” – all the photons are passing through the body.



“Grey bodies” – part of the photons are absorbed and the rest of them reflected.

4

photon

interaction

annihilated

absorption

transmission
conserved

specular reflection
diffuse reflection

Fig. 4. Types of interaction between radiation and matter (Image: Fraunhofer ISE, Freiburg,
Germany; Solarpraxis AG, Berlin, Germany).
Simplified, it is possible to make a connection between the brightness and the capabilities of the
bodies to receive energy as heat, Table 2.

brightness impression

interaction

energy transfer

white

predominantly reflection

slight

grey

reflection and absorption

medium

black

predominantly absorption

large

Tabl
e 2: Brightness and heating up of objects.
The absorption properties of the different materials are very important in order to use them to
receive the energy from the whole solar spectrum, Table 3.

5

designation

definition

black

complete absorption of all photons in the solar
spectrum

white

complete reflection

grey

uniform but incomplete absorption of all photons

selective

absorption of photons from certain spectral ranges,
reflection and/or transmission in other parts of the
spectrum. Thus, every coloured object absorbs
selectively.

Tab

le 3: Absorption properties of different materials.
Solar radiation on the Earth's surface
In order to analyse the radiation as a phenomenon, it is very important to know the quantity of the
solar energy on the Earth’s atmosphere boundary and Earth’s surface. The solar radiation can be
treated as intensity of the energy flow, or so called “flux” – energy per 1 m 2 surface for 1 second.
The dimension is W/m2 or kW/m2 and the solar flux intensity is called “irradiance” or this is the
power of the solar radiation. On the Earth’s surface the behaviour of the irradiance varies
according hours of the day, day and night, seasons and weather conditions. Even within a date of
the year, the irradiance can vary because of different weather conditions and from this point of
view, the irradiance often has stochastic character. The energy caused by the irradiance is called
irradiation. Its dimension is Wh/m2 or kWh/m2 and it represent the integration value over a period
of time, Table 4.
physical quantity

name

units

power

radiation intensity (irradiance)

Wm-2, kWm-2

energy

radiation energy (irradiation) Whm-2, kWhm-2

Table 4: Units to measure radiation.
The extraterrestrial radiation to the Earth’s atmosphere is in the boundaries 1320 W/m 2 – 1420
W/m2. It is assumed the radiation on the Earth’s surface to be known as “Solar constant” and its
value is
Eo = 1367 W/m2.
6

After transmission of this radiation through atmosphere, its value is reduce by absorption,
reflection, scattering and about 1000 W/m 2, max 1200 W/m2 incident on the surface of the Earth
at midday when the sky is clear. The components of this global radiation are :


Direct ( beam ) component – coming directly from the Sun.



Diffuse component – reflected from the bodies around. When the sky is cloudy, only the
diffuse component is present, Table 5.

The radiation is function of the latitude and longitude and the Sun position during the day and
seasons weather. The largest values of the irradiance are observed at midday during June and
July. The winter months are with minimal irradiance, connected with Sun and Earth motion.

sky

weather

clear blue sky

global irradiance 600 – 1000 Wm-2
diffuse fraction

10 – 20 %

hazy/cloudy, sun overcast sky, dull
visible as whitish
day
yellow disc
200 – 400 Wm-2

50 – 150 Wm-2

20 – 80 %

80 – 100 %

Table 5: Radiation intensity for various weather conditions (Image: Fraunhofer ISE, Freiburg,
Germany; Solarpraxis AG, Berlin, Germany).

It is considered the diffuse radiation in Central is within the range of 40% (May) to 80%
(December) of the global [1]. This fact is very important when building projects based on solar
energy exploitation.

7

Fundamentals of solar energy exploitation
In order to convert as much solar energy as possible with minimal loses the systems must answer
some requirements. Especially the requirements are important for the receiver, i.e. the body
which has to absorb the solar energy and to transfer it :


The receiver should absorb solar energy from the whole spectrum without loses.

For this purpose the absorbers are made with special covering ready to absorb as much as
possible energy and to reduce the reflection to minimum.
The quality of conversion may be expressed with the efficiency, 

Useful energy transferred

= 
Radiation on receiver surface
There is one more factor for a better performance of the system and it is connected with the
energy income. This is the receiver orientation to the Sun. There are two types of systems –
stationery and tracking. The first is oriented to a fixed position with two angle parameters –
azimuth and tilt. The azimuth angle in the Northern hemisphere is oriented to South and the tilt
angle is from horizontal surface to the Sun. For the different locations optimal tilt angle has been
obtained, fig.5. The tracking systems are similar to sunflower. They are single axis (moving to
azimuth) and two-axis (moving to azimuth and up to the Sun). They are more efficient, but also
more expensive.

8

Section of the sky, from
which radiation is
incident on the
receiver

Fig.5. Diffuse radiation on a tilted receiver (Image: Fraunhofer ISE, Freiburg, Germany;
Solarpraxis AG, Berlin, Germany).

The maximal sola gain can be realized if the surface of the receiver is normally oriented to the
radiation. For instance in Central Europe small tilt angle is preferable, fig.6 [1] :

summer

winter

Fig.6. Direct radiation on a tilted receiver (Image: Fraunhofer ISE, Freiburg, Germany;
Solarpraxis AG, Berlin, Germany).
The optimal tilt angle is connected with the system requirements, Table 6 :

9

boundary conditions

angle

maximum total annual energy yield

30°

optimisation for winter months

60°

good performance in spring and autumn

45°

Table 6: Receiver tilt angle for different conditions (in Central Europe).

Obviously the minimal tilt angle must be on the equator and the maximal on the poles.
The other important questions are about the absorber’s area and the energy storage capacity –
thermal or electrical in standalone systems. Both parameters must be calculated very carefully
based on observations and measurement of solar energy parameters due to the local
characteristics of the site where energy conversion system is provided. The absorber area and
storage capacity are crucial for answering the energy consumption needs during the whole year,
day and night.

2. Data collection technical systems for measurement of solar energy
parameters
The main parameters of the solar energy are solar irradiance, W/m 2, i.e. the solar energy flux
intensity, the ambient temperature, oC and duration of solar shining in days. The practical
measurement of irradiance is provided by technical devices, most precise are so called
“pyranometers” and different constructions of silicon sensors. In fig.7 pyranometer production of
Kipp & Zonen, type CM11 is shown. The CM11 pyranometer is intended for high accuracy total
global, or diffuse sky, solar radiation measurement research on a plane/level surface. The CM11
is fully compliant with the ISO-9060 Secondary Standard pyranometer performance category
(highest ISO performance criteria for a pyranometer). Extremely high mechanical tolerances are
maintained during manufacture to ensure optimal measurement performance in the field [2].

10

Fig. 7 Kipp & Zonen CM11 pyranometer
Most convenient for complex technical systems for meteorological measuremnents are the silicon
sensors, fig 8. The LI-200 Pyranometer is designed for field measurement of global solar
radiation in agricultural, meteorological, and solar energy studies. In clear, unobstructed daylight
conditions, the LI-COR compares favorably with first class thermopile-type pyranometers, but is
priced at a fraction of the cost.

Fig.8. LI-COR 200 silicon sensor for irradiance.
The systems for data collection include different sensor for solar irradiance, temperature,
atmospheric pressure and relative humidity, data logger and data transfer system. The data logger
registers records and stores the measured data. The row data may be transferred by different
technical decisions – Radio Frequency transfer (RF), Bluetooth and post popular and efficient for
long distances – GPRS with SIM cards communicator. This approach is very suitable, because
the data may easy received by e-mail. After the data transfer, the data can be treated different
software.

3. Methodology for Solar Potential Assessment

11

The assessment of the solar potential is very important for all projects, concerning exploitation of
solar energy. The solar radiation has stochastic behavior. It is not a constant value and depends on
the site position, meteorological conditions, local weather conditions and other factors. In order
to assess the solar potential on a specific site, the theory has not a precise answer. They are
theoretical methods for calculation the different types of radiation, in this number direct and
diffuse, but these methods do not calculate the local weather conditions, which are dependent of a
lot factors and they cannot be predicted. Something more, during the same seasons, the weather
change itself so that on a fixed date of the year the solar irradiance can vary within wide range.
In order to obtain a preliminary value for the yearly solar gain, the solar irradiance or the
irradiation have been statistically observed on different sites and solar maps built. Also it is
possible to observe via satellite the weather condition and this is also opportunity for a local solar
assessment. Unfortunately these methods do not describe correct the solar irradiance on a specific
place during the year where a solar energy conversion facility is planned for building and the
project cost are in hard dependence with the solar gain.
One of the most popular systems for solar irradiance and irradiation prediction for specific
geographical coordinates (latitude and longitude) is so called PVGIS. This is Photovoltaic
Geographical Information System which includes a data base for interactive access to the solar
potential for different geographical coordinates. The data base includes solar data for a period of
25 years. Based on the solar data, a lot of so called solar maps are charted. The maps represent
data long-term yearly averages of selected climatic parameters. The original data (with 1-km grid
resolution) have been aggregated into files with grid size of 5 arc-minutes [3]. On the following
figures, two solar maps are presented. The first is for the global irradiation on horizontal surface,
fig.9, the second for global irradiation on optimally-inclined surface, fig.10.

12

Fig.9. Global irradiation on horizontal surface, kWh/m2, PVGIS.

Fig.10. Global irradiation on optimally-inclined surface, kWh/m2, PVGIS.

13

For preliminary assessment the solar potential a PV estimation utility is available [4]. The utility
is interactive and by latitude and longitude give, some solar parameters are calculated, fig.11.

Fig.11. Screen of the GIS interactive maps.

For instance, following coordinates are used in order to calculate the monthly solar irradiation :
44°24'8" North, 26°0'56" East.

The resulting window shows the following :

Monthly Solar Irradiation
PVGIS Estimates of long-term monthly averages
14

Location: 44°24'8" North, 26°0'56" East, Elevation: 83 m a.s.l.,
Solar radiation database used: PVGIS-classic
Optimal inclination angle is: 34 degrees
Annual irradiation deficit due to shadowing (horizontal): 0.0 %
Month
Hh

Hopt

H(90)

Iopt

T24h

NDD

Jan

1330

2150

2200

63

-1.3

579

Feb

2040

2910

2640

56

1.9

420

Mar

3140

3850

2930

43

6.1

330

Apr

4480

4870

2960

28

11.8

114

May

5640

5580

2790

15

17.7

15

Jun

5990

5660

2560

8

21.5

1

Jul

6100

5910

2770

12

23.6

0

Aug

5530

5860

3260

25

23.0

5

Sep

4260

5220

3730

41

17.5

79

Oct

2890

4160

3690

54

12.3

279

Nov

1500

2310

2270

61

6.3

461

Dec

1100

1780

1840

64

0.2

597

Year

3680

4200

2800

34

11.7

2880

15

Hh: Irradiation on horizontal plane (Wh/m2/day)
Hopt: Irradiation on optimally inclined plane (Wh/m2/day)
H(90): Irradiation on plane at angle: 90deg. (Wh/m2/day)
Iopt: Optimal inclination (deg.)
T24h: 24 hour average of temperature (°C)
NDD: Number of heating degree-days (-)

PVGIS © European Communities, 2001-2010
Reproduction is authorized, provided the source is acknowledged

As seen of PVGIS grid resolution – 1 km, the correct data may only be obtained by solar
parameters measurement on the very place where energy conversion facility has to be built.
Something more, it is necessary to perform the measurement during a period of one calendar
year. Then the results may be compared with the statistical GIS data in order to make a precise
preliminary comparison between investment and solar gain.
For solar parameters measurement a different sensors and systems for data management and
collection are used.
As a result, the following procedure for solar irradiation may be performed :

1. After data collection, the solar irradiance must be integrated to find the irradiation for a
given day.

2. After all days of the month irradiation calculation, the monthly irradiation can be
obtained.

3. The same way after summarizing the monthly irradiation, the whole energy for the year or
the solar potential for the period of measurement is known.
The following example shows the realization of the procedure.
16

Example
Measurement values for 9-th of June 2008 by the data logger are given. The measurement is
performed on a horizontal surface. The data format is as follows :

Time / Irradiance, W/m2 / Time / Ambient temperature, oC

17

3:30:00 AM

0

3:30:00 AM

14.2

3:37:30 AM

0

3:37:30 AM

14.1

3:45:00 AM

0

3:45:00 AM

13.7

3:52:30 AM

0

3:52:30 AM

13.6

4:00:00 AM

0

4:00:00 AM

13.6

4:07:30 AM

1

4:07:30 AM

13.6

4:15:00 AM

3

4:15:00 AM

13.8

4:22:30 AM

7

4:22:30 AM

13.8

4:30:00 AM

11

4:30:00 AM

14.1

4:37:30 AM

16

4:37:30 AM

14.3

4:45:00 AM

21

4:45:00 AM

14.3

4:52:30 AM

24

4:52:30 AM

14.3

5:00:00 AM

28

5:00:00 AM

14.3

5:07:30 AM

32

5:07:30 AM

14.4

5:15:00 AM

36

5:15:00 AM

14.8

5:22:30 AM

39

5:22:30 AM

14.9

5:30:00 AM

44

5:30:00 AM

15.3

5:37:30 AM

47

5:37:30 AM

15.6

5:45:00 AM

50

5:45:00 AM

15.9

5:52:30 AM

56

5:52:30 AM

16.1

6:00:00 AM

216

6:00:00 AM

16.4

6:07:30 AM

242

6:07:30 AM

16.9

6:15:00 AM

182

6:15:00 AM

17.3

6:22:30 AM

290

6:22:30 AM

17.8

6:30:00 AM

316

6:30:00 AM

18.4

6:37:30 AM

338

6:37:30 AM

19

6:45:00 AM

365

6:45:00 AM

19.6

6:52:30 AM

390

6:52:30 AM

20.3

7:00:00 AM

416

7:00:00 AM

20.9

7:07:30 AM

439

7:07:30 AM

21.6

7:15:00 AM

466

7:15:00 AM

22.4

7:22:30 AM

495

7:22:30 AM

23.1

7:30:00 AM

518

7:30:00 AM

23.7

7:37:30 AM

542

7:37:30 AM

24.2

7:45:00 AM

563

7:45:00 AM

24.6

7:52:30 AM

590

7:52:30 AM

24.9

8:00:00 AM

609

8:00:00 AM

25.4

8:07:30 AM

632

8:07:30 AM

26.1

8:15:00 AM

653

8:15:00 AM

26.6

8:22:30 AM

674

8:22:30 AM

27.3

8:30:00 AM

697

8:30:00 AM

27.6

8:37:30 AM

718

8:37:30 AM

28.4

8:45:00 AM

735

8:45:00 AM

29.3

8:52:30 AM

758

8:52:30 AM

30.1

18

The data from the logger must be time integrated in order to find the irradiation from the irradiance.
The time interval of the logger is 7 minutes and 30 seconds, i.e. 7,5 minutes or 7,5/60 hours.

The irradiation can be expressed as follows :

(1),
where


E is irradiation, Wh/m2.



I is irradiance, W/m2.



to, tn are the starting and final time of integration, hours.

(2),
where



k is the 1-st measured irradiance value, n – the last one.
– time step, hours, i.e. data logger time interval.

The precision of ( 2 ) is good enough, because if the rule of trapezoids is implemented, the only error
will be at the beginning and at the end of integration. As seen all the values in the midnight are zero, so
they will not affect the integral value.
Finally, the sum of irradiance data is 66266 W/m2 and the time interval is 7,5 / 60 hours = 0,125 hours.
The irradiation is equal to 8283 Wh/m2 for the 9-th of June.
In the figures below, the measured data for irradiance and temperature are presented.
19

Solar Irradiance, W/m2

Fig.12. Measured irradiance, W/m2 for 9-th of June.

20

Ambient temperature, oC

Fig. 13. Measured ambient temperature, oC for 9-th of June.

For comparison, the irradiance and ambient temperature for 14-th of February are given. June and July
are the months with highest irradiance and the winter months with lowest.

Solar Irradiance, W/m2

21

Fig.14. Measured irradiance, W/m2 for 14-th of February.

Ambient temperature, oC

Fig.15. Measured ambient temperature, oC for 14-th of February.

22

The irradiation for 14-th of February is 1681 Wh/m 2. Measuring ambient temperature is very important,
because of days of freezing and the days with high temperature. In both cases the efficiency of the solar
cells drops down, freezing lowers the optical characteristics and high temperature lowers the cell’s
efficiency.
After the monthly irradiation data has been calculated, the sum gives the potential, fig.16.

Solar irradiation, Wh/m2

Fig.16. Total potential for the year : 1330 kWh/m2

4. Basics of solar thermal conversion
These systems for solar thermal conversion collect and store solar energy as heat. They produce
heated water for domestic consumption and heating. The main elements of the solar thermal
systems for water heating include collectors, storage tanks and plumbing systems. Other type of
solar thermal systems produce directly heat air for space heating, [5]. Other area of
23

implementation of solar energy is solar refrigeration based on adsorption and absorption
processes, but still not widely developed due to the high price of the equipment.
Solar technologies are widely spread for the last 30 years, but known for a thousand of years. The
applications of solar thermal conversion may de described generally as [6] :


Solar water heating (thermosyphon, integrated collector storage systems, air systems,
direct circulation and indirect water heating systems).



Solar space heating systems (water and air systems).



Solar refrigeration (adsorption and absorption systems).



Industrial process heat systems (low temperature air and water based, steam generation
systems).



Solar desalination (direct solar stills, indirect conventional desalination).



Crop drying.



Solar thermal power generation systems (parabolic trough systems, the power tower or
central receiver systems, parabolic dish systems, Stirling engine).

A classification of the applications is proposed in [7]. This classification is performed by the
temperature range of the systems :


Low temperature.



Medium temperature.



High temperature.

A large amount of the solar thermal facilities are working within the temperature interval < 43 oC.
The heat transfer medium is liquid or air and these systems are used for domestic needs. The
medium interval includes range 60-82 oC. The working media is also liquid and air. They also can
be used for domestic needs and small industrial equipment. The high interval is for temperatures
more than 82 oC. In order to obtain high temperatures, the facilities are concentrators like mirrors
24

or lenses. The media is transferring heat to other media (most often water) in order to create
superheated steam for electricity and heat production. In order to perform 24 hours operation, the
systems use heat exchanging elements to store heat.
Another classification gives collectors distribution based on their performance. They can be :


Flat solar collectors for liquid media or air. Can be glazed or unglazed.



Evacuated tube solar thermal collectors with direct flow or heat pipes.



Concentrating collectors with parabolic concentrators, power towers or solar cookers.

5. Technologies development of solar thermal installations
The flat solar collectors for liquid media are usually glazed for better performance. They are
cheaper than evacuated and have good cost performance ratio. By the German Solar Society
(2005) a glazed flat plate collector can collect approximately 450 kWh/m2 per year.
Flat collectors, fig.17 consist of the following basic elements :


Casing with insulation of the back side.



Glass Cover.



Absorber plates with pipes.

25

Fig.17. Flat plate solar collector principle scheme.

The solar radiation passes through the Glass Cover and is heating the absorber plates with the
pipes. Then by heat conduction in absorber and pipes and following convection the heat is given
to the fluid in pipes causing rising of its temperature. In order to reduce convection heat loses, on
the back side of the casing thermal insulation is provided.
The next step of the solar thermal technologies is implementation of evacuated tubes with direct
fluid flow or for better performance with heat pipes. The construction of these collectors uses
better evacuation between the Glass Tube, fig.18 and the inner tube with the fluid inside. The
evacuation is provided in order to decrease the heat losses from the inner tube. For a better
performance also a Reflector is provided causing better collection of the solar radiation to the
inner tube placed in the focal point of the Reflector.

26

Fig.18. Heat pipe evacuated tube collector

The typical positioning of the solar collectors is roof incorporated, fig.19 or standalone
construction, fig.20.

Fig.19. Roof incorporated flat solar thermal collector

27

Fig.20. Thermosyphon system with water Tank (Photo courtesy of SUNSYSTEM).

The thermosyphon shown in fig.20 needs proper mechanical system for mounting. The
performance is better due to the possibility of mounting the surface of the absorber with the
optimal angle “” for the latitude of the geographical site, fig.21.

28

Fig.21. Mechanical system for optimal angle performance.

The solar water domestic heating systems cannot use directly water flowing through the pipes of
the flat solar collectors. The reason is freezing during cold periods which may cause damage of
the system. For this purpose some thermal agents like Propylene Glycol and others in so called
“Active Indirect Systems” is used, fig.22.

29

Fig.22. Active Indirect Solar Thermal System

In order to prevent damage or incident during system’s exploitation special precautions have to
be taken. The reason is the vessels under pressure. On the other hand electronic equipment also is
needed in order to control optimally the system’s operation.
In fig.23 an example of such system is given. The elements for a normal work of the system are :


Air vents for prevention air in the pipeline.



Pressure Relief Valve for prevention system of exploding due to high pressure.



Expansion Tank for regulation the pressure of the system. The thermal fluid expands with
the rise of temperature and in a closed system the pressure also rises.



Sensors for control and optimal operation.



Controller which operates the circulation pump when the solar irradiance is not sufficient
to heat the thermal agent.

30

Fig.23. Active Indirect Solar Thermal System with equipment for control and operation. (Picture
courtesy of U.S. Department of Energy. Energy Efficiency and Renewable energy).

6. Task, function and characteristics of the solar thermal installations

Main task – to transform the solar energy into heat.
Function as an exchanger and thermal storage.
Characteristics :



Collectors with air flow.



Collectors with liquid flow.



Glazed collectors.
31



Unglazed collectors.



Evacuated with direct flow.



Evacuated with heat pipes.



Collectors with concentrators :
-

parabolic through;

-

compound parabolic;

-

dishes ( parabolic most );

-

power towers;



Stationary collectors.



Tracking systems :
-

single axis,

-

two axis.

7. Function and requirements of solar storage units
Storage units serve the purpose of a “battery” that separates the heat input of solar radiation from
the user's energy consumption. Since the temporal pattern of energy input usually does not match
that of energy consumption, this separation is necessary in all but a few solar-thermal systems.
The time period of storage varies between a few hours, days or (in case of seasonal storage),
months and strongly depends on the consumption side of the system (e.g. swimming pool,
potable water or room heating) and the desired solar fraction [8].
The goal is to store the available solar energy as completely as possible during periods of low
power demand and later supply this energy as efficiently as possible when needed.
Requirements on the solar storage unit:
•

high heat capacity of the storage medium,
32

•

good heat efficiency (small tank surface and good insulation),

•

a thermally well stratified structure of the tank filling,

•

a desired life cycle of 25 years (or more) for the complete system,

•

low cost and good availability,

•

tank and storage medium must be compatible with the environment and hygiene (e.g.
potable water)

•

the system must withstand the expected range of pressures and temperatures.

Due to its good heat capacity, environmental-friendliness and availability, ordinary water is the
commonly chosen storage medium. It is possible for domestic water systems to use potable water
directly as the storage medium, or, if an auxiliary solar buffer and heat exchanger is used, non
potable “heating water”.
Alternative systems (latent-heat concepts and chemical storage) have entered experimental
development and will not be described further at this point. Non of the surveyed systems used
storage media other than water.

Types of Storage Units construction :
•

Portable water storage.

•

Zink galvanized Storage Tanks

•

Enamel-Coated Tanks.

•

Stainless Steel Tanks.

•

Solar Buffer Tanks.

•

Lightly Pressurized Buffer Tanks.

•

“Combi” Storage Units.

•

Open-System Buffers.
33

•

Seasonal Storage.

Storage Tank insulation
The heat losses of conventional storage tanks are very significant. In order to keep heat
losses of solar storage units within reasonable limits, special requirements must be met for
insulation, fig.24 :
• a small surface to volume ratio of the tank,
• closely attached insulation,
• complete insulation of the tank, including upper and lower sides,
• insulation of all pipe connections and fitted/ accessories,
• pipework guided into the storage tank or below the insulation,
• avoidance of in-tube circulation (heat losses caused by convection within the pipe).
The size of the solar storage tank depends, of course, on the size of the collector array, and also
on the daily hot water consumption during the week (e.g. approximately constant consumption on
all week days, or only consumption on work days, e.g. in factories, workshops etc.).

34

Good insulation of outlets

Poor insulation of outlets

Fig.24. Storage Tank insulation

The aim is for the solar storage tank to store, on an intermediate basis and until needed, the solar
heat collected during the day and not directly passed to the consumer. Simulation calculations
with variation of the solar storage tank volume were carried out for location in Cologne,
Germany, for a solar installation with a hot water consumption in the building of 7m 3/d. The
calculations were carried out for two types of building, for a residential building with
approximately constant consumption on all week days and for a workshop with no consumption
on Saturdays, Sundays and public holidays. In addition, the system load for both buildings was
changed (70l/(d.m2) and 40 l/(d.m2)) by increasing the collector area from 100 m 2 to 170 m2. The
calculations presumed that the storage tank volume can be accommodated in one single container.
In practice it is often necessary to install two or three storage tanks. This would lead to a slight
increase in storage losses, so that the degree of utilization of the system would be slightly less
than the values calculated. The results of these simulation calculations are shown in Fig.25.
35

Consumption profile apartment house,
specific load 70 l/(m2.d)
Consumption profile apartment house,
specific load 40 l/(m2.d)
Consumption profile workshop no weekend
demand, specific load 70 l/(m2.d) on working
day
Consumption profile workshop no weekend
demand, specific load 40 l/(m2.d) on working
day

10 20

30

40

50 60

70

80

90 100

Specific buffer volume [l/m2 collector area]
Fig.25. Degree of utilization of the solar installation as a function of specific storage tank volume
(volume per square meter of collector area), for two buildings used differently and for two
specific loads respectively (medium quality solar installation components). (Courtesy of [8])

8. Power systems with solar thermal collectors
Solar power plants are built in places, where constant and high solar potential is observed. The
largest plants are in California, Nevada, Spain, France, China, India and others. Usually they are
based on Rankine cycle and it can be simplified on the following fig.26 :

36

Fig.26. Chart of Rankine cycle with preheated water vapor

The following pictures show different projects with the characteristics mentioned above :

37

Fig.27. Dish concentrator, multisegmented

Fig.28. PS10, Sevilla, Spain, 11 MW, 624 Heliostat mirrors, 115 m height of the tower

38

.
Fig.29. Solar Two, California. ( Photo courtesy of NREL ).

39

Fig.30. Evacuated tube collector field in Tibet, China.

Fig.31. Mojave Desert, California, 354 MW, 936384 mirrors, Area 6,5 km2

40

Fig.32. Nevada Solar One, Las Vegas, 64 MW, 180000 mirrors, 1,6 km2

9. Management and exploitation of Solar Thermal Systems
9.1 Flat Solar Collectors Efficiency calculation
The heat transfer analysis is used in order to assume the efficiency of a flat solar collector [6], [9].
Steady state balance expressed for the incomes Qin and loses Qloses :

,W
The component

(1)

comes from the solar radiation
,W

(2)

where


is the transmittance of the glass cover,



– absorptance capability of the absorber’s plate,



I – Solar irradiance, W/m2.
41



– absorber’s plate area, m2.

The heat loses can be expressed by :

,W

(3)

where




U is heat transfer coefficient, W/(m2.K),
– temperature of the plate, oC,
– ambient temperature, oC.

The heat power calculated for the collector fluid :

,W

(4)

where


G – mass flow rate of the collector’s fluid, kg/s,



Cp – specific heat of the fluid, J/(kg.K),




– outlet temperature of the fluid, oC,
– inlet temperature of the fluid in collector, oC.

The efficiency can be calculated by :

42

, or

.

(5)

Example 1
A flat solar collector has the following parameters : Area 2 m 2, tramsittance-absorptane product :
) = 0,82, Heat transfer coefficient – 6,5 W/(m 2.K). The global solar irradiance

(

to collector’s surface is 720 W/m2. The ambient temperature and the absorber’s plate temperature
and respectively 28 oC and 49 oC. Calculate collector efficiency.

Solution

From (5)
,

0,63.

( The difference between Kelvin and Celsius degree has same value. )

9.2 Air Solar Collectors
Air as a fluid is not suitable for heat exchange. The reasons are the low heat capacity and
viscosity. On the other hand most of the modern air condition facilities are heating or cooling air
for accommodation needs. The realized idea is to heat air by sun radiation and supply it directly.
They are two directions of solar air collectors : building integrated and standalone collectors like
these heating liquids. The standalone collectors have almost the same construction – glazing,
43

frames with insulation and absorber. In this case there is outside air driven by a fan, heating the
air in collector and supplying the heated air in accommodation. In fig.33 a standalone air
collector is shown. The test parameters of the collectors are :



Active area – 1,25 m2.



Orientation – South.



Time of test – November, 10:30 AM.



Tilt angle to the Horizon – 60 deg.



Air mass rate – 76,6 kg/hour.



Inlet air temperature – 16 oC.



Outlet air temperature – 56 oC.



Global Solar irradiance – 800 W/m2.



Heat Power of the collector – 676 W/m2.

The advantages of this type air collectors are :


Low weight. There are not pipelines under pressure.



No freezing or boiling processes of the air.



Lack of corrosion materials.



Low price of the collector.

44

Fig.33. Prototype of a standalone air solar collector

Example 2
Air solar collector has mass flow rate of 70 kg/hour. The air heat capacity is 1,0 kJ/(kg.K). The
inlet temperature of the air is 5 oC. On the outlet is 34 oC. Calculate the heat power of collector.

Solution
From (4) -

,

G = 70, kg/hour = 70/3600, kg/s = 0,0194 kg/s.
Cp = 1,0 kJ/(kg.K) = 1000 J/(kg.K).

Q = 0,0194 . 1000 . ( 34 – 5 ) = 564 W.
45

9.3 Practical example of Solar Thermal System with SCHOTT Evacuated Tube
Collectors ETC 16.



Construction of SCHOTT Evacuated Tubes, fig.34 :

Fig.34. SCHOTT Evacuated Tube ETC 16 with casing, [10]

46

Fig.35. SCHOTT Evacuated Tube ETC 16, opened, [10]

47



Parameters of SCHOTT Evacuated Tube ETC 16 :



Scheme of the project :

48

Fig.37. Solar thermal collector and thermal system

Legend :
-

A0 – automatic air vent;

-

ЦП – circulation pump with electronic temperature control;

-

Д – Expansion Tank with safety valve, 50 l;
49



-

Б – water heater (2 serpentine type), 500 l;

-

АУ –automatic control unit. Т1 > Т2 – pump working temperatures;

-

ПВ - water steam safety valve, 3 bar;

-

Т – thermometer,

-

М – manometer;

-

Ф – water filter;

-

ВК – one-way valve;

-

В – closing valve;

Simulation of the system by Polysun 3.3. Results.

By Polysun 3.3 the system has been simulated. The detailed and graphical reports are presented
as follows :

50

51

52

53

54

10. Physical basics of solar electricity conversion
Solar cells convert light into electric energy. Very general, this is a three-step process which can
be explained by means of fig.36 :

negativ electrode
n-doped silicon

p-n junction

positiv electrode

p-doped silicon

Fig.36. Cross section of a crystalline silicon solar cell (Image: Solarpraxis AG, Berlin, Germany).



Absorption of light, delivering electrons in an excited state.



Locally separation of positive and negative charges.



Conduction of the charges to an external circuit.

Absorption: In most cases the absorber is a semiconductor, and the used transitions are
interband transitions. The excited state comprises an electron in the conduction band and a hole
in the valence band. These interband transitions are characterised by a sharp long-wavelength onset of the absorption ("band-edge") and a relatively broad absorption band. So-called "direct"
semiconductors have a very high absorption coefficient in their absorption band. In this
wavelength regime light is absorbed within a few µm of penetration depth. These semiconductors
are therefore well suited for thin film solar cells, with a thin semiconductor layer on a low-cost
55

substrate. "Indirect" semiconductors, like the common crystalline silicon, have a much lower
absorption, and about 150 µm of material are needed to absorb most of the light. As a
consequence, relatively thick (200 to 300 µm) sheets of silicon – so-called "wafers" – are the
main material for the production of solar cells. Other absorption mechanisms are also used for
photovoltaic devices. One well-known example is the well defined absorption of light in organic
dyes leading to excited molecular states of the compounds.
Charge separation: Electrons and holes in semiconductors are separated by diffusion or by drift
of the charge carriers in the space charge regime of a p-n-junction or in a hetero junction of two
materials. Other separation mechanisms are also known, e.g. the tunnelling of electrons through
very thin insulating layers. – In the already mentioned dye cell the charge separation comprises a
charge transfer of the excited electron to the conduction band of a wide band-gap semiconductor,
e.g. TiO2.
Conduction of the charges: The charge separation leads to the generation of a voltage between
the both sides of a solar cell. Contacts have to be applied to conduct the charges to an external
circuit. This is not always easy since the contacts should have a very low contact resistance to
avoid electric losses within the device. For semiconductors the right selection of contact
materials, combined with the proper technology, may lead to "ohmic" contacts with very good
conductance. It is obvious that at least one of the contacts must have a high optical transmittance
to allow light to reach the absorber inside the device. One means is to use thin conducting oxides
(TCO) as transparent contacts, the other technique is to use small metal fingers ("grid") on top of
the cell, tolerating some shadowing (4 to 7 %).
Optimisation of the electric output of a solar cell requires a good coupling of the light to the
absorber layer, a high absorption within this layer, and a low recombination of the generated
carriers before they are properly separated. The common means to reach this goal are the
application of antireflective coatings (ACR) or surface texturing, the use of high purity
semiconductors, and the passivation of the surface of the semiconductor. As a consequence, solar
cells of acceptable efficiency require elaborate technologies. The development of materials, of the
design of the cells, and of the production technologies is in a steady process of optimisation, and
this process is far from being completed.

56

The Solar Cell as a Power Generator
Since the solar cell is intended to deliver the highest possible amount of electric power (at a
certain level of incident light) it is useful to analyse the I-V-characteristic with respect to the
power which can be extracted. If the operating point of a solar cell is shifted along the I-V
characteristic curve (by changing an external load resistor) the electric power generated in the
external load can be found by multiplying the current by the voltage in the operating point. This
has been done in the graph in fig.37. The power that can be extracted goes through a maximum at
a certain point, the Maximum Power Point (MPP). It is characterised by PMPP, the maximum
power, by IMPP, the current at maximum power, and by VMPP, the voltage at maximum power.

maximum power point MPP
3.5
3.0

PMPP

1.2

c ell c u rren t [A ]

IMPP

A
V

1.4

ISC

2.5

1.0

2.0

0.8

1.5

0.6

1.0

0.4

0.5

o u tp u t p o w e r [ W ]

STC:
T = 25°C
AM = 1.5
E = 1000 W/m2

0.2

VOC

0
0

0.1

0.2

0.3

0.4

cell voltage [V]

0.5

0.6

0
0.7

VMPP

Fig.37. Power generated by a solar cell as a function of the operating point (Image: Solarpraxis
AG, Berlin, Germany).

57

0.6

3

0.4

2

0.3
0.2

1

short circuit current [A]

open circuit voltage [V]

0.5

0.1
0

0
200

400

600

800

1000

irradiance [W/m2]

Fig.38. Dependence of VOC and ISC on the light intensity (Image: Solarpraxis AG, Berlin,
Germany).

All electrical parameters of a solar cell are depending on the intensity and the spectrum of the
light as well as the temperature of the solar cell. The dependence of current and voltage on the
illumination level are shown in fig.38. Whereas the current of the cell is linearly dependent on the
irradiance, the voltage and the MPP are not, and therefore the description of the behaviour of a
cell under different illumination level is complicated.
For different levels of irradiance but constant cell temperature, this leads to a set of characteristic
curves as shown in fig.39.

58

3.5

module current [A]

3
2.5
2
1.5
1
0.5
0
5

module voltage [V]

10

15

20

25

range of VMPP

Fig.39. I-V-curves of crystalline silicon module at different irradiance levels and constant
temperature (Image: Solarpraxis AG, Berlin, Germany).

11. Solar Cells, Modules and Photovoltaic Systems
Depending on the technology used, a single solar cell generates a MPP voltage of approx. 0.5 to 2
V. Hence, electrical loads can rarely be run directly at this low voltage unless they are small
devices or toys. In general, a higher voltage is necessary. It can be provided by arranging multiple
cells in series, as is done with batteries. For instance, 36 crystalline silicon cells are connected in
series in standard modules, producing a MPP voltage of approx. 18 V, which is suitable to charge
12 V lead acid batteries. In the meantime, there are standard modules with 72 or more cells, and
special modules consist of up to several 100 cells connected in series. In turn, several such solar
modules can be connected in series - a “string” - to form a solar generator that produces voltages
up to some hundred Volts.
To ensure the desired solar generator output power, several modules or strings can be connected
in parallel, thus increasing the current. This modular interconnection allows photovoltaic
generators to be designed with outputs from milliwatts to megawatts - all with the same basic
technology.
59

In the following, the properties of cells and modules connected in series and in parallel will be
discussed, particularly their response to partial shading. Then, some typical structures of solar
modules will be illustrated and items like lifetime and recycling are presented.
Solar cells and solar modules are connected in series to produce greater overall voltages.
In a series connection, the current is the same in all of the cells so that the overall voltage - as

A
V

c e ll c u rre n t [A ]

shown in fig.40 for three similar cells - is the result of the sum of the individual voltages.

ISC
3
2
1

UOC
0

0.4

2 UOC
0.8

1.2

3 UOC
1.6

2.0

cell voltage [V]

Fig.40. Series connection of three similar solar cells and their current-voltage (I-V) diagram
(Image: Solarpraxis AG, Berlin, Germany).
Connecting solar cells and modules in series does, however, have one major drawback: the
“weakest link” determines the performance of the whole array. Even if only one cell is partially
shaded, this cell determines the overall current, and hence the output of the whole string.
Partial shading thus has to be avoided to the extent possible !
Even small shaded areas such as from poles, cables, tree tops, leaves, bird excrement, and dust
lead to large output losses and are usually the cause of unsatisfactory energy yield of
photovoltaic systems.

60

The same is true of series connections of cells with different characteristics due to production
tolerances or if parts of cells within a module have broken off and hence become inactive. Here,
too, the weakest cell determines the overall performance. Cells and modules should be selected
and put together so that they produce nearly the same MPP current. This additional work pays for
itself when the energy yield increases!
If the system has to produce greater currents, the modules or strings can be connected in parallel
as shown in fig.41. In parallel connections, all of the cells have the same voltage, and the overall
current is the result of all of the individual currents.

9

3 ISC

cell current [A]

8

V
A

7
6

2 ISC

5
4
3

ISC

2
1

VOC
0

0.1

0.2

0.3

0.4

0.5

0.6

cell voltage [V]

Fig.41. Parallel connection of three similar solar cells and their current-voltage diagram (Image:
Solarpraxis AG, Berlin, Germany).
As with a series connection, the question is how the shading of a single cell or module will affect
performance. In general, the effects on the energy yield are somewhat lower than when the same
number of solar cells are connected in series. In particular, there is not usually a danger that the
shaded cells will be overloaded by reverse current from other cells in typical arrays. There is thus
no need to use string diodes with strings connected in parallel according to IEC 60364 if the
modules used are protective class II and their no-load voltage deviates by less than 5 % from each

61

other - conditions that solar power systems generally fulfill. In low-voltage applications such as
solar home systems etc., string diodes can generally be omitted.
The wiring diagram and the resulting current-voltage characteristic curves for a solar generator
with several solar modules connected in series and in parallel are shown in fig.42.

g
e
n
e
r
a
t
o
rc
u
r
r
e
n
t[
A
]

1

2

3

3 ISC
3

2 ISC

2

ISC
UOC

2 UOC

3 UOC

generator voltage [V]

1

n
u
m
b
e
ro
fp
a
r
a
lle
lc
o
n
n
e
c
te
ds
tr
in
g
s

number of series connected modules

Fig.42. Modules connected in series and in parallel and their current-voltage characteristics
(Image: Solarpraxis AG, Berlin, Germany).

12. Electrical systems with photovoltaic power

62

A photovoltaic system to power an appliance or small consumer generally consists of a
photovoltaic generator, a charge regulator, a storage battery, and voltage regulator, fig.43.
In very small systems such as those in watches and pocket calculators, the photovoltaic
generator may only consist of one or a few solar cells. To provide greater output, the individual
solar cells are wired in solar modules.
charge regulator

PV generator

voltage regulator

DC load

storage battery

Fig.43. Principal design of a photovoltaic system to power an appliance or small consumer
(Image: Fraunhofer ISE, Freiburg, Germany).

In addition to small modules to power specific products and applications, standard modules are
manufactured to provide greater energy supply. These standard modules are dimensioned so that
they have a nominal voltage of around 15 to 17 V are thus able to charge batteries with nominal
voltages of 12 V.
When selecting the modules, the module’s output and voltage have to be correctly dimensioned.
Furthermore, the module has to be mechanically constructed to withstand the weather and
climatic conditions in the long run. Depending on the available space and the kind of integration,
the geometric dimensions and the physical properties and attachment options of the module’s
frame may also play a role.
To prevent the storage batteries used from being overcharged or deep discharged, a charge
regulator is used between the photovoltaic generator, the battery, and the load. The charge
63

regulator generally also contains a discharge protection diode that prevents the battery from
discharging over night through the photovoltaic generator. A good charge regulator consumes
very little power and has a low voltage disconnect that protects the storage battery from being
deep discharged.
The storage battery stores the energy produced by the photovoltaic generator and makes it
available to the consumer during bad weather or at night.
Appliances that get their power from photovoltaics mostly use nickel-cadmium batteries (NiCd)
or nickel-metal-hydride batteries for storage. However, lead batteries, lithium-ion batteries, and
capacitors (called double-layer capacitors) are also used.
In small systems powered by photovoltaics, lead batteries are usually used. Thus, special models
of car batteries with extra thick lead plates (called solar batteries) are used for mobile
applications, for instance to power electric consumers in campers, boats, and weekend homes. In
photovoltaic systems to power homes with permanent residents and daily charging/discharging
cycles, usually tubular plate (“OPzS") batteries are used. They have deep cycles and hence long
service lives. Sometimes, normal car batteries are used in Solar Home Systems because they are
more readily available and cheaper.
For some applications, maintenance-free lead batteries are useful; their electrolyte is captured in a
fleece or gel. These batteries have 100 times less sulfuric acid vapors than lead batteries with
fluid electrolyte, which allows them to be installed in the same housing or space as the
electronics. Maintenance-free batteries do not leak and can thus be run in any position.
A voltage regulator may be required to adapt the voltage of the photovoltaic system to the
voltage of the consumer. For devices powered by photovoltaics, this regulator is usually a DC/DC
transformer, which transforms one direct current into another.

PV generator

voltage
regulator

consumer

64

Fig.44. Principal design of a photovoltaic system without energy storage (Image: Fraunhofer ISE,
Freiburg, Germany).
When demand and supply occur at the same time for systems to power appliances and small
consumers, the storage battery is not needed, fig.44. Some examples of these consumers are
pocket calculators, fans, and water pumps.
Photovoltaic systems to supply off-grid consumers far from the grid with medium and large
output
If greater output levels are required or if conventional household appliances or industrial devices
are to be used, the system voltage should be 230 V~. To attain this voltage, an inverter is added
to the system, fig.45. The inverter transforms the direct current produced by the photovoltaic
generator or the storage battery into alternating current.

charge regulator

PV generator

inverter

house grid

storage battery

Fig.45. Principal design of a photovoltaic system with alternating current output (Image:
Fraunhofer ISE, Freiburg, Germany).

At our latitude, an energy supply based exclusively on photovoltaics requires large photovoltaic
generators due to the fluctuations in solar radiation. The same is true of photovoltaic systems that
have to have great availability. Hence, a mixture of generator types is generally combined to form

65

hybrid systems. Combining photovoltaic generators and motor generators ensures the same
power security as in the public grid, fig.46.

PV generator

charge regulator

storage
battery

motor
generator

inverter

house grid

battery charger

Fig.46. Principal design of a hybrid system consisting of a photovoltaic generator and motor
generator (Image: Fraunhofer ISE, Freiburg, Germany).

If solar radiation is good, the photovoltaic generator can meet the whole energy demand in an
environmentally friendly way, with no emissions and no noise. Excess energy is stored in
batteries. At night or during bad weather, batteries cover the energy demand. When the battery is
in danger of being deep discharged, a motor generator – such as a diesel or liquified petroleum
gas motor – is started to cover power needs and recharge the storage batteries at the same time.

66

PV generator

charge regulator

wind turbine

battery charger

motor
generator

battery charger

storage battery

inverter

house grid

Fig.47. Principal design of a hybrid system consisting of a photovoltaic generator, wind turbine,
and motor generator (Image: Fraunhofer ISE, Freiburg, Germany).

In windy areas, a wind turbine can be added to the system, fig.47.
As the photovoltaic generator and the wind turbine compliment each other very well when the
design is correct, the motor generator’s operating hours are reduced, thus also lowering the
consumption of fossil energy.
In the future, fuel cells will also be used in PV hybrid systems. If users want to be completely
independent of fuel supplies and have a wholly autonomous system, an electrolyser and a
hydrogen storage system are integrated, fig.48. The electrolyser is operated in the summer when
the photovoltaic generator produces excess energy and the lead batteries are fully charged. The
hydrogen created is stored and is available for the fuel cell in the winter.

67

PV generator

charge regulator

inverter

house grid

storage battery
fuel cell

charge regulator

hydrogen storage

electrolyser

system

Fig.48. Autonomous power supply from a combination of solar generator and yearlong hydrogen
storage system (Image: Fraunhofer ISE, Freiburg, Germany).
Grid-connected photovoltaic systems
Photovoltaic systems can feed power to the public grid via an inverter. The advantage is that no
energy storage is required as the energy can be used elsewhere, i.e. the output of conventional
power plants is reduced. The share of solar energy fed to the grid is currently small, but future
energy scenarios forecast an important role for solar energy. State subsidy campaigns (such as the
100,000 Roof Program in Germany), rates for the power fed to the grid high enough to cover
costs, and legal frameworks (such as the Renewable Energy Act in Germany) promote the release
of such systems on the market.

68

If compensation for the electricity fed to the grid from small, distributed systems is lower than the
price for the electricity from the public grid, the inverter feeds the power directly into the house’s
lines, fig.49.

house grid

PV generator

inverter
energy fed
to the grid

energy
consumption

utility
Fig.49. Small, distributed photovoltaic system that feeds power into the houses lines (Image:
Fraunhofer ISE, Freiburg, Germany).

On sunny days, it provides power to the consumers in the house, while excess power is fed to the
grid and metered. If the weather is bad and at night, the house gets its power from the
conventional grid. The output of such systems is approx. several kWp.
If the compensation for the solar power fed to the grid is greater than the purchase price for
electricity from the grid, the system is generally designed as shown in fig.50.

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House
grid

PV generator

inverter
energy fed
to the grid

energy
consumption

utility
Fig.50. Small, distributed photovoltaic system that feeds power directly to the public grid (Image:
Fraunhofer ISE, Freiburg, Germany).

EVU
Hausnetz

The modular character of photovoltaics also allows for large, grid-connected power plants to be
built. They feed power directly into the medium or high voltage grid via an inverter.

utility
PV generator

Bezug

inverter

Fig.51. Principal design of a grid-connected photovoltaic power plant (Image: Fraunhofer ISE,
Freiburg, Germany).
Photovoltaics in distributed grids
The deregulation of the European energy market means that the energy sector is becoming more
competitive for the generation, distribution, and trading of electricity. New actors will enter the
market thanks to the open access to power grids and appropriate conditions for third-party use of
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such grids. They will strengthen the current trend toward the distributed generation of electricity.
In the process, photovoltaic systems can become part of new supply structures. Fig.52 shows an
example of what the distribution structure could look like in the future with distributed electricity
generation systems.

nuclear power plant

large hydro power
plant

coal fired power
station

high voltage grid
medium voltage grid
low voltage grid

PV

fuel cell

house grid

storage
battery

house grid

PV
generator

wind power

large PV
generator

CHP
(combined
heat and
power unit)

Fig.52. Principal design of a future energy supply system with distributed electricity generation
(Image: Fraunhofer ISE, Freiburg, Germany).

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13. Computer design and simulation of photovoltaic systems
User requirements for software tools can vary considerably. In the case of many standard system
configurations it is sufficient to look only at the system energy flow, or perhaps to just have
design assistance for the coordination of individual components. Many inverter manufacturers
offer tools that specifically address these needs. These tools largely prevent some of the more
primitive dimensioning errors during the coordination of PV modules and inverters in gridconnected PV systems. On the other hand, there are many cases in which detailed time-step
calculations over the course of at least an entire year are absolutely necessary. In the case of gridconnected PV systems, these situations often arise as a result of partial shadowing of the
generator surface. With stand-alone systems, for example, is often a question of the security of
supply during the winter; with hybrid systems, it is usually a question of economic optimization
or the technical fine-tuning of components. According to these different requirements, the
software can be divided into categories.
Dimensioning tools: This category includes, for example, utility programs from inverter
manufacturers. These programs allow users to tune the PV generator to a chosen inverter. The
programs base their calculations on predefined maximum values for the generator‘s operating
temperature and the resulting electrical maximum values for the voltage levels. Using such
programs should eliminate the occurrence of serious error messages in grid-connected PV
systems. These tools can generally be downloaded for free from the websites of the leading
inverter manufacturers (for example: SMA, Fronius, Mastervolt, and Siemens).
Programs for system simulation: Solar power system simulations, in other words the
reproduction of PV systems with the help of a computer, require good statistical and dynamic
descriptions of the individual components, which need to be accurately linked to the complete
system. Moreover, detailed system simulations require calculations in small, temporal
increments. For energy yield calculations of PV systems, an hourly basis is an established. In the
meantime, almost all of the simulation programs on the market are included in this group.
Open simulation environments: Yet users who frequently conduct system simulations using the
above tools quickly come to recognize their limitations – for grid-connected PV systems,
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naturally, less than with hybrid systems. With hybrid systems, users often find that the predefined framework supplied by the program does not allow them to simulate certain
configurations. This is especially true when connecting multivalent systems – for instance, the
connection of building-integrated PV systems at the level of electrical energy, thermal
requirements, and daylight use. Whereas programs like InselDi or Modes can tackle particular
problems with multivalent systems, it is often more flexible to use open simulation environments
like MATLAB / SIMULINK. With these programs, users can implement almost everything
themselves. In fact, they have to, since these programs have very limited direct access to block
descriptions or libraries.
Databases and utility programs: Although databases and utility programs do not directly fall
under the category of system simulation software, some of them play an important role in
detailed calculations. The most prominent example in this category is the climatologic database
Meteonorm – an important tool for the field of irradiance data.
Evaluating the Simulation Results
For grid-connected PV systems, a bad system design quickly results in a poor economic
performance. For remote, off-grid systems, a bad design can lead to either system breakdown or
to rapid battery ageing. By using simulation programs, such mistakes should be avoided.
In any case, the user must be able to use the program correctly as well as to be aware of potential
sources of error. Input errors and the associated erroneous calculations are not uncommon, even
for experienced users, with programs having complicated input interfaces.
Several programs, like PV*Sol and PVS 2001, perform a plausibility check of the most important
input parameters. Even with this check, however, all errors can not be excluded with certainty.
This is why system planners should always validate the simulation results with values gained
from experience. For grid-connected systems, the Performance Ratio (PR) or the annual energy
output divided by the peak power in kWh/kWp are very good references. These parameters are
employed for comparison by most programs.
In Germany the Performance Ratio of the PV system should be greater than 70%. The annual
energy yield should be at least 700 kWh/kWp. When the tilt angle of the roof is unfavourable or
when shading occurs, the annual energy yield can be less. If simulation results are more than
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1000kWh/kWp, or clearly less than 500kWh/kWp for a facade integration or less than
700kWh/kWp for a standard roof integration, then it can be assumed that the input parameters are
wrong.
Checking the validity of the simulation results for autonomous systems is, however, more
difficult. System planners can check the results using rules of thumb or by comparing them to
values gained from experience from PV systems which already exist.

14. Applications of photovoltaic systems
Most of the grid-connected photovoltaic systems in Europe belonging to private system operators
are installed as small systems on the roofs or integrated into the roofs of existing houses.
Mounting or integrating photovoltaic systems onto or into house roofs or walls is an attractive
solution, as double usage is made of the building envelope, and the additional costs for support
structures are less (example in fig.53).

Fig.53. Small grid-connected, roof-mounted photovoltaic system (Photo: ZSW, Germany).
Fig.54 shows the schematic structure of a small, grid-connected photovoltaic system. The cables
from the individual photovoltaic modules all lead to the generator connection box, which can also
contain appropriate protection diodes and surge protective devices. If one inverter is installed for
74

each string of photovoltaic modules, the generator connection box is not required. The inverter
converts the direct current (DC) from the photovoltaics into alternating (AC) single-phase or
triple-phase current. The AC current is then fed into the grid of the local utility or power grid
operator. (In the text which follows, "utility" will be used to refer to both the utility company and
the power grid operator.)
Large, free-standing photovoltaic systems often have an installed power of at least 50-100 kWp
up to the megawatt range, fig.55 and generally feed the electricity into the medium-voltage grid.
Their main applications are for industry, municipalities or companies in the power supply sector.
Depending on the location and system configuration, the systems can be operated not only to
supply electric power but also to meet peak loads or stabilise voltage levels. The spectrum of
system operators includes utilities, industrial companies and local government authorities. The
trend specifically in Germany is toward very large roof-installed photovoltaic systems up to a
rated power of two megawatts and free-standing arrays up to four megawatts.

2

1

5

photovoltaic generator
generator connection box
inverter
meters
grid connection and fuses

1
2
3
4
5

photovgener1
generator
oltaic atorbo
inver
connection
x 3
met
ter
2 4
grid er 5

4

3

Fig.54. Schematic diagram of a small, grid-connected photovoltaic system on a private house
(Image: Solarpraxis AG, Berlin, Germany).
75

Fig.55. Free-standing, large grid-connected photovoltaic system ("Sonnen", in Germany) with a
rated power of 1.75 MW (Photo: Voltwerk AG, Hamburg, Germany).

Fig.56 and fig.57 show configurations for feeding electricity into the public and an intermediate
grid.

1

3
2

4

5

1
2
3
4
5

photovoltaic generaotor
house connection box
DC cabling
inverter
meters

76

Fig.56. Configuration for feeding electricity directly into the public grid (Image: Solarpraxis AG,
Berlin, Germany).

2
1

3

4
Example
1
2
3
4

solar meter 100 kWh
total consumption
1000 kWh
consumption meter 910 kWh
delivery meter 10 kWh

Fig.57. Configuration for feeding electricity into an intermediate grid (power transit) (Image:
Solarpraxis AG, Berlin, Germany).

15. Management and monitoring of exploitation of photovoltaic systems
Grid-connected photovoltaic systems have achieved a high quality standard. Depending on the
type and the manufacturer, performance guarantees for 15 to 25 years are now given for
photovoltaic modules. Thus, they represent long-lived investment goods, which can operate well
for 30 years or more, independent of the formal guarantee.
Inverters have developed from manually constructed, individual units to mass-produced industrial
products, which leave little to be desired concerning their technical standard and efficiency.
Nevertheless, the inverter still has weaknesses concerning its long-term reliability. Development
work is primarily directed toward improving reliability and lifetime.

In practical system

operation, the lifetime can already be influenced positively by choosing a "cool" position for the
inverter.
It is highly recommended to include system monitoring or control in systems with several
independent subsystems, as reading the value from one central supply meter is a very inaccurate
77

method of diagnosing a fault in a single subsystem. An obvious approach is to use the data
logger or monitoring device offered as an option by various inverter manufacturers.
The typical, specific energy yield of a grid-connected photovoltaic system depends, among other
factors, on site-specific characteristics, the quality of the components used and the care taken
during installation.

Fig.58 shows the typical energy flow diagram for a grid-connected

photovoltaic system. The ideal energy yield would be obtained if the incident solar energy were
converted to electricity according to the specifications on the module data sheet. However, other
factors also play a role in a real system.
The basic principles of the photovoltaic conversion process mean that the module efficiency
decreases logarithmically with decreasing irradiance, and reaches a value of zero when it is
completely dark. Depending on the frequency distribution of the irradiance in the module plane,
the reduction amounts to about 4.5 %. The reduction due to soiling is strongly dependent on the
location. Industrially polluted regions must reckon with larger values than the 2.5 % indicated
here. The type of mounting affects the operating temperature. Free-standing systems or roofmounted systems with very good ventilation of the back surface experience a temperaturedependent annual reduction of about 3.5 %. However, this value is significantly greater for roofintegrated systems with limited back-surface ventilation.
The choice of the cable cross-section and the additional installation material used (switches,
separators, fuses, etc.) causes circuit losses which increase quadratically with the current. When
the cable cross-section is being chosen, a voltage drop of 1 % in the circuit has become accepted
as a compromise between investment costs and energy losses. The possible mismatch between
individual strings seldom causes a problem, as systems are usually constructed of modules from
one production batch.
The losses due to the inverter have two components, the losses involved in the direct DC/AC
conversion and the losses resulting from the non-ideal setting of the operation point at any time.
Finally, when the losses on the AC side and in the meter are also taken into account, the real
amount of electricity fed into the grid is obtained.
Specific energy yields of many systems have been determined within long-term monitoring
programmes. To allow comparison between different systems, a universal quality criterion has
been introduced, which depends only weakly on the location. The so-called "Performance Ratio
78

(PR)" is the ratio of the real energy yield (e.g. electricity fed into the grid) to the ideal energy
yield. In the example shown in fig.58, this is

PR [ % ] = 100  E real / E ideal

The performance ratio can also be represented as follows:

PR [ % ] = 100 

specific electricity yield [ kWh / kW ]
solar radiation in the module plane [ kWh /m² ] / 1 [ kW /m² ]

PV generator with rated power P = 1 kW p
at Standard Test Conditions
deviation from rated
module efficiency value
and G < 1000 W/m²
module soiling
module temperature >25°C
shading
mismatch and DC losses
MPP mismatch error
inverter losses
AC losses, meters

79

Fig.58. Energy flow diagram for a grid-connected photovoltaic system (rated power of 1 kW,
located in Northern Germany) (Image: Solarpraxis AG, Berlin, Germany).

Recently installed systems at good locations in Germany achieve specific energy yields ( =
energy yield relative to the installed electric power rating) exceeding 930 kWh / kW. The
Performance Ratio of good systems is then well above 70 %. The PR value is particularly
informative if the ideal energy output or the specific energy yield is based on the measured rated
power of the modules. Otherwise, of course, it is easy to influence the PR value in the desired
direction by the "choice" of the reference value.
As an example, the operating results of a well-functioning, small photovoltaic system (1 kW) are
presented in fig.59 by the evolution of the monthly PR over 3.5 years. The free-standing
generator on a flat roof is oriented to the south and has a tilt angle of 32 °. The increased
temperatures during the summer months cause a slight reduction of the PR, as do the reduced
irradiance values in winter.
A further example is given by fig.60, which shows the PR of a 100 kW grid-connected
photovoltaic system from Switzerland, which started operation in 1989 and has been monitored
in detail. The results obtained have been very good on average; operation was disturbed in
2000/2001 due to inverter defects.

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Performance Ratio [ % ]

Month

Fig.59. Monthly performance ratios over 3.5 years for a recently installed, grid-connected
photovoltaic system (1 kW) in Northern Germany. Operation commenced in January, 1998.

Performance Ratio [ % ]

(Data Source: IEA Performance Database, Image: W. Knaupp, Stuttgart, Germany).

Month

Fig.60. Monthly performance ratios over 13 years for a 100 kW, grid-connected photovoltaic

81

system in Switzerland. Inverter difficulties occured in 2000 and 2001 (Data Source: IEA
Performance Database, Image: W. Knaupp, Stuttgart, Germany).
If the planning is good, the components are appropriately selected and the installation is carefully
carried out, grid-connected photovoltaic systems operate extremely reliably for many years. Also
taking the low maintenance demands into account, they present a promising option for power
supply, now and in the future.
Management of PV Systems
The management of PV systems is focused on setting the stage of successful PV installation,
based on safety and economical analysis. All the components of the installation safety plan have
to be identified, also analysis of different utilities interactions, commissioning plan, cost and
financial analysis of the system have to be performed.
Main activities :


Critical safety issues of DC power circuit, especially DC arcing and arc-fault issues.



Permitting and inspection processes for PV systems.



Common metering technologies and rate structures for PV systems.



Different types of utility connection agreements.



Commissioning process of PV systems inspection, maintenance, monitoring and
successful troubleshooting.



Introducing the sources of costs, associated with PV systems.



Methods for calculation of investment return.

Example
The PV system parameters are :


Analyzed period – 1 year.



Measured solar irradiation – IR = 120 kWh/m2.
82



Solar generation area – A = 10 m2.



Efficiency factor of the PV modules –  = 15 %.



Electrical energy exported to grid – Egrid = 110 kWh.

Calculate performance ratio ( PR ).
Total PV plant output is
E = IR . A = 120 . 10 = 1200 kWh. This is the maximal available energy for measured
irradiation. The nominal output is :
Enominal = E . kWh.
The performance ratio is :
PR = Egrid / E nominal = 110 / 180 = 0,61 or 61 %.

REFERENCES

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[1] SOLTRAIN Study materials, Volumes A,G,C, Technical University of Varna, Bulgaria, 2004.
[2] Kipp & Zonen. http://meteostanice.agrobiologie.cz/figs/cm11.pdf.
[3] PVGIS, http://re.jrc.ec.europa.eu/pvgis/.
[4] PVGIS, http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php
[5] Michael C. Baechler Theresa Gilbride, Kathi Ruiz, Heidi Steward, Pat M. Love. Solar
Thermal & Photovoltaic Systems, June 4, 2007.
[6] Solar Energy : Thermal, Mihir Sen, University of Notre Dame, November 6, 2009.
[7] USA Energy Information Administration (EIA).
[8] Peuser F.A., K.H. Remmers, M.Schnauss, Solar Thermal Systems, SOLARPRAXIS, Berlin,
2002.
[9] Duffie John A., W.A.Beckman, Solar Engineering of Thermal Processes, John Wiley & Sons.
[10] http://www.schott.com/uk/english/download/solar_thermal_rd319.pdf.

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